The Polaroid 405 back will let you use pack film like FP100c with most 4x5 cameras. It's this: https://www.catlabs.info/product/405-back
There are lots of them on the auction site, but sellers tend to ask $$$ for them, despite the fact that the film they take is functionally extinct.
Oh I should add, if your FP100c is the 4x5 variety (FP100c45) then you want the Polaroid 550 back, not the 405. FP100c45 is pretty rare these days though.