If you don't have a local source for used film boxes, post a wanted ad here on APUG, and I'm sure someone has an extra or two they can send you for the cost of postage.
If you use the zone system, a compact way to handle lots of film is to make a set of cardboard dividers for N-2, N-1, N, N+1, N+2. Put tabs down the side so you can keep them in order in the dark, and then one tab on the top of the top sheet, so you know which is the top of the stack. I like this method, and I develop by time and temperature.
Another approach is just to keep the sheets in order and keep a list so that you can sort them out later. Michael A. Smith and Paula Chamlee use this method, but they also develop by inspection, and even though Michael claims that he's never gotten a sheet out of order this way, DBI is self-compensating, so that if you did get a sheet out of order, you could adjust your development time for that sheet on the fly in the darkroom, which you couldn't do if you develop by time and temperature.
I've usually been able to find a dark closet or bathroom to load and unload on the road, but a Harrison changing tent might give you more peace of mind.