Adding glass to turtle tank

Hey everyone, I'm looking to add glass to my turtle tank. I believe it's roughly 40 gallons, a bit wider vs high. Anyway instead of keeping my little dry dock on top, and since he has ample room, I was thinking to seal in a piece of glass that's watertight to divide a small section out and then build a small ramp and sand in the dry area. Is there any recommendation on the glass? Internet is a bit spotty but a lot of things point to 20mm glass panes. Has anyone done something like this with suggestions? I'm pretty handy and capable. Just trying to snag a little advice.