Pickup cab rear panel

[Androd]

I'm working on building a LCV special with a couple of demountable bodies, based on a very rotten van I acquired a few years ago. You know the sort of thing - patches on patches and lots of over-plating rather than proper repairs.

I've got half of the cab repaired and am working on the passenger side now. The biggest problem I can foresee is that, because I want to make the rear demountable, I need to close off the back of the cab. New panels are astronomically expensive and used ones impossible to find. Does anyone know if someone makes a fibreglass panel ?

Alternatively, has anyone got an original panel in reasonable condition that I could use to make a mould ? I've owned several fibreglass cars and am pretty good with the stuff as I've had to mould repair panels before. I'd need to borrow it for a month or so.