wedge all around... now i'm thinking of a design many full body spinners have.
Sure good to deflect blows, and sacrificial HDPE is.. well... that stuff is, compared to anything else useable in that weight class really cheap. if you even shock mount the electronics, that will be one shell hard to open.
Only problem: what happens when you get upside down? you'll need to either stick the wheels out through the top (needing bigger wheels) to drive on your back if needed (but the wedge would be turned around then, too). Or get some self righting arm or such to pop up.
Well, not really need, but would sure be a good addition if something gets under you and turn you around, since as mentioned pure HDPE-wedges get blunt fast, so you don't have much hope to get under another pusher, flipper, lifter etc. Even many spinners stay low to the ground as defense against these.

Anyway, the way i see it the meta is like this for now:
big spinners, have massive energy stored and causing others to really think about their side armor.
heavy armored pushers to charge into the spinners before they spin up and push them into the pit or whatever.
flippers to get anybody who isn't low to the ground and get a self righting arm or can drive on both sides.
Hammers/Axes to remind everybody top armor is needed, too.
grabber, lifter etc. for everybody trying to have an active weapon, but maybe easier to build than these high powered ones.
Crusher... just the occasional few to remind people internal structure is needed, too.

And at least from the one event i saw that's also about how often they appear/how dangerous they are, from top to bottom.
(But that one was in germany, so might be different for you).
Anyway... something well armored and able to push definitely is competitive, if you can drive it well. And that just needs practise.
Your motors won't even have to be too powerful, since mostly traction is the bigger problem. And getting really good wheels... well, there are some threads about that, a science in itself.
So with that design:
A inner shell, as compact as possible from thin HDPE to house the electronics.
And around that the wedge with some air pockets. sounds good to me.

Possible problems:
- weight. Wedge in all directions needs a lot material or steep wedge. But can be done, if no active weapon needs weight.
- most wedges have two driving wheels somewhere in the back, and one point of the wedge (or close to the wedge) slide over the ground to stay as low as possible. could be interesting to get all sides low.
- if someone get under you, you'll be left with wheels in the air as soon as one side leaves the floor (since the opposing side gets stuck on the floor, wheels will hang in the air between them). So no way to retreat and free yourself.
- pure HDPE wedge doesn't hold up long, possibly add a steel tip to it. nothing fancy, maybe like this? http://forum.roboteers.org/gallery/u...48e3-large.jpg
- what to do when you're upside down?