Thanks.
I'd thought about simply making up a 4x5 guide frame out of L channel brass or plastic and have a couple of stretchy springs across it, top to bottom to hold in the film holders, etc. Of course, that would have meant making up a new GG holder. The nice thing about the Graflex back is that everything is guaranteed to fit to it, with minimal hassle and I already have a viewing hood that fits it. I'll probably re-inforce the top and bottom and trim the overhanging edges, next.