Probably the best option for using a monorail in the field is a Sinar F/F1/F2. They're cheap(ish) and have short rail sections that you can compress the camera down onto for transport, then just add the longer rail section(s) as needed. Better in some ways would be a Toyo VX125 but they're A: rare, B: expensive, and C: limited in versatility because of the compact folding rail system. I have a 4x5 F that I use for teaching and I can fit it on the 6" rail, a 12" rail, a couple of lenses and the other odds and ends you need in a large-ish Lowepro backpack. I much prefer my wood field cameras for travel, though.