Hello Gavin, I have been wondering about this too. Check out http://www.photobackpacker.com/home.asp , these are for 4x5 and 8x10 but I don't see why the cases that fit inside those wouldn't work for what you want. I have one of these systems, set up for a 8x10 but have since modified that to handle my Fotoman 612 two cones and a Fuji 690. I could get another case for my 4x5 and through that in to. I have found these type of bags hold far too much, look I got GAS already! AHH. Post what you come up with.
Hope this helped