It's wholesale pricing since I buy quite a bit from them, but you should have no problem finding 14 inch squares for less than 5 dollars each, especially annealed (untempered) glass. Tempered will usually be 1.5 times the price, depending on how large the suppliers kiln is, some have minimum pricing for tempered, and some will pre-drill holes before tempering (but the glass, and the holes will sometimes become distorted or shrunken).
Ordering glass on the internet is always a great way to get a terrible price. Family owned glass shops will always be a friend, and will always be willing to bend the rules to make a sale.
For what you spent on your tempered glass, I would have made and shipped a custom 14 inch cube with 7mm glass and untempered bottom :P (and for another 40 dollars would have acid etched your name or a logo into one of the sides).
But great enthusiasm! If you needed at 13x13 inch piece for your bottom I could have mailed a piece to you! I don't have any 14x14 pieces right now though. And nothing in 3/16ths thickness. I would put calls into local glass places (do not call window places, most only install pre-made cookie cutter windows, and do not know how to cut glass).
You'll have this thing done in no time! And if you remove silicone, remember that silicone doesn't bond as well to silicone as it does to glass, so use acetone or 90% isopropyl alcohol and razor blades to CLEAN the old silicone off the bonding surfaces before you try to re-silicone.