Feather spinners can be... well... i don't think there is any material left nowadays that is strong and light enough to take a good hit from one of the really good spinners and still don't take serious damage.
So it mostly depends on the angle. If your armor is 90° to the spinner, or even having some pointy bits standing out, you'll possibly get shredded. If i remember right, there were feather spinners ripping through 8mm armox. And as soon as you want an active weapon, even 3.2mm of that stuff all around might be too heavy.

So your best bet is to not give them 90° to bite into, but make it angled, so they will more "scrape" on the armor than to "bite" into it. You might even deflect a blow (mostly from the horizontal ones) so the spinner itself will get bounced and possibly having his energy backfiring at him.
Now the really hard thing is... how do you make it all angled from all sides, with the possibility to get flipped? But that's something you have to find a solution, or more likely, live with something "the best you could do".
Could also be a good idea to shock-mount the armor, but really depending on the kind of armor and all.
basically, any metal-based armor tries to deflect hits without actually absorbing any energy, and "tries" to stay as one, flat surface. Once it is bend or ripped, it will stay so and be a perfect weak spot for any hits coming after that or even locade your drive or such.
HDPE is more like "oh, you took a part out of me? so? try again, i've got some left". parts can be chipped out, but they won't be bend out leaving something for the enemy to "hook into", and won't be bend in and press against your drive or stuff like that. It will stay in shape, but get chunks get cut out of it. And that most likely with every hit, where some metals might just take a scratch, if even that.

What actually is better in a feather... depends on where and how it is used, and what you can work with.



With HDPE, you got two options:
Making the chassis from Ali or stuff like that, just enough to put all the electronics etc. together, and then put HDPE on that as the armor.
Or if you'd want to go with all HDPE, i think 20mm for the armor and 10mm for internal structures should be enough. Maybe even less for internal stuff, depends on what and where.

But: Never try to drill and tap into HDPE. The bolts just get ripped out.
For me, i do the screwing depending on if i likely need to screw/unscrew here often.
If the screws are mostly once in, never out again (like internal walls to base plate, or internal walls to other internals) just wood screws. And not just a few.
But can't do these for Top to Armor and stuff like that you'll likely want to open and close a few times (or in case of the armor: might have to swap for new parts to repair). After a few times of screwing and unscrewing you'll just have a big hole to look through, and any screw gets ripped out easily. So either take barrel nuts as mentioned above for these parts, or try to melt other, big nuts in sideways (didn't try that, but could work), or if the bot is flat enough, you could do what i do: drive-in nuts.
If it is box-shaped in any way and not too high, they work really nice. Just screw the box together with only a few wood screws (just to hold everything in place). Take a drill (ideally a drill press or a straight, steady hand) and drill 5mm holes through top plate, any walls, and bottom plate in one go. now make the holes 6mm on the first cm on one plate, and sink in the bolts from the other side. Now you can hammer in the nuts through the 6mm holes, or, what worked better for me: take one longer bolt and a washer from the other side, and just drive those nuts in. they get straighter that way, saving you trouble later on.
Take out the wood screws from the side where you sinked in the holes and not put the nuts in. I'd leave the few next to the nuts, just so not the hole chassis falls apart once you remove the bolts to open the top plate.
Might want to step it up a bit on the bolt size, i just used M5 because i had those around.

Downsides i found so far for that way:
-swapping side armor is not that easy without a drill press or having the parts pre-made. the holes have to be on the same place in the same direction.
-if the nuts don't go in straight, instead of bolting into them you could just push them out, and will have to try again. gets annoying.
- since sinked in bolts get flatter than the nuts, it could be wise to make the bottom plate removable, not the top, so you don't get stuck on the floor with these nuts. it's just 1mm or such, but could be enough. But gets a bit confusing with wiring the motors etc., and could leave you drive in the wrong direction when you turn the bot around again

In quiet some characteristics HDPE behaves like wood, so what's used to hold wood together or give wood the form you like, will likely also work for HDPE.

Anyway, it is definitely possible to make a working bot in a way you only need HDPE as material (maybe 10 and 20mm, if you have weight left more at least for side armor is better. if you stay with mostly rectangular and triangle shapes for the parts it is really easy to calculate the weight even without CAD or stuff like that. make an excel-sheet with all the surfaces of the parts added together in cm², once for the armor parts and once for internal stuff. now you can experiment and multiplicate those numbers with 0,5 to 3 or whatever. whatever you get from these numbers will be your weight in g. But don't forget screws, those are really heavy...)
Tools for this you'd only need some kind of saw (any saw will do, but some will better than others), a drill (preferably a drill press, too), maybe a few screwdrivers and stuff like that, something to put the electronics together. With some luck they're mostly plug and play, as well as some duct tape and zip tie, a knife and a hammer (you can always need those. The knife comes in handy, since you can even carve HDPE with it.) Possibly some hot glue.
You can even saw out round pieces of HDPE and screw bike tire in them as wheels.
So, with one spare sheet of HDPE in each thickness you use, you can make all the spares you might need (except for gearbox etc.) without too big machines to carry with you to an event. Big plus compared to guys who need a mill and wielding and whatever for their spare parts.

But, if you want to go anything wedge-like, you should consider make the top/bottom plate (partially) from some kind of metal. you'll just get a way sharper/sturdier edge with it. Hard to get under the enemy when you rammed the wall a few times with an edge from HDPE.