Heater Box Repair

Hi All,

Dismantling my dash for paint and pulled the heater box yesterday and noticed that one of the cable brackets that attaches to the box has pulled away and left a hole behind. I need to reattach it somehow and wondering what the boxes are made of. I could use JB Weld or a fiberglass product, but looking for help and which is better or is there something else to use.

Thanks, see damage below. Box is from my '66 Barracuda and is Model #741.