this looks amazing. lightweight, minimum of moving parts, simple, probably fast... perfect for me. i think i'm gonna start building myself one of these as soon as i get my first one up and running (need something to machine the HDPE
i don't have much experience in all this, but it seems to me it was constructed for light, soft-ish materials. i too would go with, say, MDF (25 or 30mm), heavy enough for base, but not too hard to work with. however, i'd use aluminum blocks instead of HDPE (it's just me, i like aluminum ).
as i said, i don't have much experience (none at all, actually), so it would be best to take some advice from more advanced builders and machinists.
nevertheless, i wish you all the luck with the build, and i hope you'll update the thread often!