Rear window rust repair without welding

Get you a map torch with a brazing tip and braze it. Use a good ol' coathanger. Once done undercoat the backside.

Plenty of youtube vids to show you how to braze.

Fiberglass doesn't stick to metal and will delaminate sooner than later. Depending what you are replacing, AMD makes a replacement filler panel between the trunklid and rear glass. If that is the case, buy it and glue it in with SEM panel bond. STRONGER than spotwelds!!! Just clamp it on with visegrips and let it dry.