In message , David
writes
If I make it large enough then it could form a back support when I try
and fix the plastic back together. Which will be the next problem as it
has no doubt stretched and distorted whilst breaking. I assume
fibreglass may be the way to go, or car body filler.

For something like that plastic repair, I'd always use epoxy, possibly
with the metal support one side and glass fibre in the resin at the
back.

This, of course, assumes it doesn't need to flex a bit in use.
--
Bill