This is a tricky thing to do. Trying to dark load the FP-100 4x5 into a 545 holder might require a bit of cardboard engineering, but why not just use a 550 holder, and the 4x5 pack film? The Fuji PA-45 holder is similar to the Pol 550, and works the same. Both will fit into any 4x5 spring back I have here, and also fit into Grafloks.