Go polycarb all the way.
I was a staunch "no plastic" policy person on waterblocks. I spoke to my machinists about it and learned that all the cracking blocks were mostly made using acrylic based plastics.
We did some tests on polycarb. Tapped an M3 hole (~1/8") into some 1/4" polycarb, and drilled a 3mm hole through some 4mm thick aluminium plate and stuck the lot in a vice and used a cap head allen key bolt to tighten the aluminium onto the polycarb to test the strength of the polycarb.
The polycarb thread did eventually strip, but only after the bolt had bitten into the aluminium by almost a millimeter deep.
Even after the polycarb thread had stripped there was no way you could pull the two apart by hand as the polycarb just "bounces" back into the thread again.
We did the same test on acryclic and it just cracked as soon as the thread got even remotely tight.
After that I decided that polycarb was an acceptable substitute for copper/aluminium. Super-strong and highly crack resistant under stress.
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