[stephen low]

Why not use polyester resin and e glass (white woven cloth - 6oz)?

Fillers are designed for filling and not glueing, though they have inherent strength they aren't normally used for glueing purposes as you are proposing.

At least with cloth and resin you could bond the steel work into one part of the door first, the outside skin I presume, and let it cure and then bond the other door skin to the whole arrangement.

The beauty of this two stage assembly is that it might help with fitment of speakers into the door skin and you don't have to worry whether the steel work will move if you werer doing it all in one go.

And though less important in this application, fillers are heavy compared to resins, even resins thickened by micro-spheres to also be used as fillers.

I have assumed your body is made using polyester resin. If it is made with epoxy resin you'de need to use epoxy glues for the process - most unlikely I suspect.

Good luck