How about making that rear glass out of 1/4" thick clear acrylic sheet flat stock.
Looks like that window would be flat. Can duct tape together a few cardboard sheets and make a rear window pattern out of that. Then transfer that onto the Acrylic Sheet for size, then cut it out with a jig saw.
Seal the fresh cut window into the car with the Urethane Black Window-Weld Sealer from 3M for strength to hold it in place, not using the old hard window rubber.
If you think it needs more center support after the Urethane Dries (guessing it won't) you can use the 2 inside strap braces like the stock car racers use for their front windshields.
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Glass companies can cut flat laminated glass to fit too if it is completely flat. It is safety glass.
