We used 1/2" acrylic.
We drilled our own holes but TAP did the cutting and 1/8" bevels. We did not countersink the screws per the advice of Russ Miller at TAP. Countersinking in a sheet that big could potentially set up stress fractures with thermal expansion. The old countersunk milky "glass" that we were replacing didn't show any problems but the darker glass would get hotter and the milky glass could also have hidden small cracks.
They are heavy to ship but at least ours were really well packed so there were no problems.
-Sven