I did something similar to this once. What I did was cut the plexiglass to the exact inside measurements of the tank, then butted it up against one end, using an aquarium grade silicone to glue it to the glass. Don't use anything but the aquarium grade silicone, anything else will kill 'em all. If you can, drain your tank and keep it empty for a full day for your silicone to cure. If not, keep it supported. This worked fine for me. I then placed a couple small rocks up on top. I tried putting gravel on there but it just got kicked off. I had this one in a 29 gallon tank that I was keeping my softshell in, so I ended up putting a little lip up on it to keep gravel up there so he could bury himself in the shallow water, but I dont' think you'll need it.
Hope that works. If you dont understand ask me and i'll do my best to clarify it for you.
-chris
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The reason mainstream thought is thought of as a stream is because it's so shallow. -Unknown
Battling ignorance one stupid person at a time.
Current snakes:
1) Female Licorice Stick Black Ratsnake - Lola
2) Neonate Black Ratsnake (Het for Lic Stk's) - Frankie
3 & 4) 2 Bairds Ratsnakes - Rosie and Jose