Well take your time finding out what you should do, there's no rush to get this done quickly (even if you want it done quickly), you don't want to do a half-assed job and then have that come bite you in the butt later when the tank is full of water & rocks.
Here's the problem with simply putting a piece of glass on it with silicone, while that would easily work for an acrylic tank, usually the bottoms of glass tanks don't sit flush with the stand, as a result that would probably introduce a flexing point which I would not trust 200+ gallons water + rock, etc. Now if the bottom did sit flush (rimless tank?) then you probably could get away with simply "patching" the hole (at least as a cheap alternative) but I don't know how that would work over time.
My advice, give the Leemar guys a call (it's their type of tank right?) they might have a patch job type solution that doesn't involve a complete replacement of the bottom pane, but if that is the case, I'd replace the pane.