For 4x5", spare boxes for different batches of film are the easiest solution. I like Grafmatics as a compact method of carrying a decent amount of film, without being tied to the brands that come in Quickload/Readyload.

When I'm traveling with 8x10", spare boxes would get too bulky, so I bring envelopes with opaque plastic bags, such as are used to hold 100 8x10" sheets of photographic paper (you can buy spares in bags of 5 from B&H, packaged by Delta) and cardboard inserts. I mark the envelopes for the film type and development plan, and I also have one or two extras for groups of sheets that I want to be sure to develop in one batch (like a multi-frame panorama). You could just as well do this with any format.

I change film in bathrooms and closets, wherever I'm staying.