I just did mine and used 3/8 Plexi non tinted. I over sized the holes, counter sunk the heads and used 1/2 round. I have screws every 4" with the corners pulled back 2 inch from the edge of corner (this is where it cracked before with the 1/4 inch plexi). I then used butyl tape that was 1/4 thick. The screws are just snug, not tight. No leaks so far and its raining cats and dogs right now.
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