Sorry for delay in reply. HDPE is really good for press fitting like you said, especially if the holes are undersized. For Léim Thart, I needed to press in a 1/2 inch bearing, and for that I just drilled a 12.5mm hole and used clamps to press it in place, worked very nicely. Haven't experimented too much with different drill bits, have mostly used twist bits because I have them in the most sizes, but I'd imagine brad-point bits work better on average, at least for placing the hole. Holesaws are good for larger holes, but if you want to drill holes larger than drill bits but with a cleaner finish then I'd recommend spade/flat wood bits. Every hole I've used them for has had a perfect finish, though it seems you can only get them for sizes below an inch. Certainly worth investigating, have used them for holes for motors in beetles and for the main lifter hinge point on Barróg Doom.

Cheers for all the compliments! EWC is looking really good so far, nicely made build diary, and it's great to see the evolution of the design and lessons being put into practice.