I have seen this done and I can't recall where. I think when I got my first tranatula (which was arboreal, not burrower) I was thinking of doing the same thing. Carlton's suggestion to check with dart frog folks might be a good try. They often rig up false bottom tanks and I think that's what gave me the idea in the first place. Check out their forum and see if anyone has done it.
If it is not going to hold water you could just get a glass cutter to remove the bottom panels of two and then secure them together with clips and epoxy. If you don't want to take out the glass yourself you might be able to get a glass company to do it cheap, but it shouldn't be hard. Then you can file/sand down the edges and make sure all the glass is removed. To be on the safe side you could put a bead of aquarium sealant along the inside edges (where glass was).
I don't know...I'm just thinking aloud! LOL!! 
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0.1 veiled - Luna
0.2 felines - Kyndra and Líta
1.0 African Clawed Frog - Skipper
0.5 Mad. Hissers
0.1 Chilean Rose Hair Tarantula (no name yet)
?.? Goliath Bird-Eater Tarantula (no name yet)