Do not over-squish your O-ring. They are designed to seal with very little squish. My water blocks have a 0.062" (1.575 mm) diameter o-ring. The channel is 0.05" (1.27 mm) deep and 0.08" (2.032 mm) wide. You need to make the channel wider so that the o-ring has somewhere to bulge. The nature of the rubber in the o-ring will keep it sealing. Squishing too much can cause the o-ring to split. It can also bulge out of the channel and become pinched causing it to tear. My blocks have been tested to 130 PSI of air pressure submerged with no leaks, and should seal to at least 500 PSI. 130 PSI is all I had to test with. Of course they will never see pressures anywhere close to that.
I noticed someone suggesting that one should stay away from clear lids because they eventually leak. I think this statement is ignorant. The reason these lids DO leak is because of the poor design. Most of the lids are secured to the copper by 4 screws (one in each corner) and have the 4 mounting holes beyond the copper. This is a recipe for disaster. All the pressure that is applied to the edges of the clear lid will cause it to flex in the middle where it is not secured to copper. This is why they leak. The other problem is people over tightening the lids and cracking the plastic. The lids only need to be tightened enough so that the screws don't back out (except in flawed designs like this one).
As you can see with my design, there is no stress applied to the clear lid like the other designs. Also I have 6 screws equally spaced to ensure a good seal.
This is just my opinion of course, feel free to add your $0.02 if you disagree.