Yeah, that'll work. Jon Fettig is familiar with Delrin, you might want to ask him.
The choice of plastic here may depend on your sealing solution: if you use a gasket (RTV silicone) then your choice is wide. If however you choose to machine a groove for an o-ring, your options are going to have to include the "machineability" of the plastic. A minor issue, but still something you might want to be aware of...
I'm not finished with my block, but I opted for the polycarb sample sheets from McMaster: they're 6" by 6" and 1/4" thick, which is fine just the way it is (but I'm still going to try to double it to 1/2") and I picked up IPS Weld-On #4, which I have yet to open... (it's in a box, somewhere...
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