[frizzen]

Maybe some kind of ultra high molecular weight plastic? You'd probably spend more and longer trying to find a plastic that's stable enough at pressure. Looks like the RC systems keep it under 500psi, so it's not too crazy, but trying to figure out forces doesn't sound fun.

Can your tools run Brass?

Maybe go Aluminum for the valve body, but i might be concerned about galling if you run aluminum for both the block and spool though.
Might be able to add some teflon sheeting at the faying surfaces without very much redesign, then surface finish in the block wouldn't be quite as critical.

Might want to check around your local machine shops, if you've got a small one that does one-off stuff they could be a big help. But you also might find a production valve system price is close to theirs anyway...

*not a machinist at all. I just have decent grasp on theory, and operate some high pressure stuff*