I would say bring it to a glass shop - they can cut a piece for you to fit.

Wood is about the last thing you want to use for the cage bottom if you can help it. Cage sides or back, fine, but bottom, no.

Alternative is expanded PVC. You may find a piece large enough at a sign shop. You can order online (look for "sintra" on the web or "expanded PVC sheet". Styrene sheet is another good material for this app.

If you have to go wood, plywood is fine. You can seal it with bar top epoxy before placing it in the tank. Then silicon it really well at the seams.
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Mark