Padipro:I stand corrected.
I figured I had gotten the correct information from the folks that sold it, I guess not.
The 1/4 inch stuff I used in the past was used on a 6 inch diameter PVC pipe so the surface area was farily large. When the latches were locked it would warp the edges enough where the slightest imperfection in the O-ring sealing area would cause the seal to be lost. I switched to 3/4 inch thick for the lens and had no trouble with warpage and the seal was good.
No problem. I also used a different method for mounting my lenses that took any of the clamp stress totally off the lenses and put it all on the PVC. Here are some pics.
http://www.scubaboard.com/gallery/showgallery.php?cat=533