Fry's, our local no-service totally cheapo huge electronics giant, carried small plastic toolboxes for several years. I bought fifteen of them and made a set of cubbyholes for them, so have several categories of goodies in the back of my van, organized. On a good day (we haven't had such a good day for a few months, but the potential is there).

I avoid carrying extra of those phone parts and plates as much as possible. The other approach is to organize the other end, where you order as many as you need and that's it...with a slight bit of slack in case you screw one up. Of course, I am within driving distance of my suppliers, and they will ship if I can get two days ahead of the actual usage date.