Most of the time the reason that poly tops crack is that people try and do things to them that they weren't designed for. I know this from personal experience.

DangerDen accidentally sent me a P4 block with no mounting holes so I tried to drill the holes myself instead of sending it back like I should have.
I left the poly top on while I drilled the holes because the poly top already had holes drilled in the proper position and I thought it was strong enough to handle the drills vibrations. It wasn't

. After I was finished drilling the block looked fine, but two hours later there were cracks all around the barbs. Overnight the cracks had doubled. It would have leaked for certain if I had been running it in my system because there was a crack running directly from the outermost barb to the edge of the block.
After seeing how brittle these poly tops really are I will only go with copper topped blocks from now on.
BTW Danger Den ships internationally now so why can't you get a copper top from them?