We use a table-saw (with a wood cutting blade) and it works a treat. No fluid needed or anything, though the spray can be quite... uncomfortable, so wear suitable clothing. It's a pretty simple one so we're a little limited angles wise but the cuts shown in your pic would be easy enough. If worst comes to worst, whip out the file and surform!

As Mario said, though, HDPE won't hold a thread. Under stress the threads will just strip. You can use nut inserts or a barrel nut method, which is what we're doing for our machine. Not sure how much 10mm will like 5mm bolts, I'll let someone who has built like that answer.

You could always use L brackets and screw sideways into the material?