If you were feeling handy, you could try drilling a hole in the center of the glass at the bottom of the cage and then putting a raised second floor (plastic/pvc/abc sitting on a 1/2" riser) in the bottom that angles into that hole
You'd need to make sure of a few things first though.
1) The glass can't be tempered glass or safety glass
2) You need special bits for glass (buy them at a hardware store)
3) You need to go slowly and keep drilling slightly larger & larger holes to avoid breaking the glass.
I'm sure there are good tutorials on the internet on how to drill glass.