I made mine with a 4"x10' stick of schedule 40 pvc. One end got a glued cap, the other end got a female clean out fitting with a sunken head brass plug screwed in. When I travel, the sunken head makes it impossible (nearly) to unthread without the proper tool. Just my .02.
This ^^^^^^ You could even go up to 6" if you wanted to. Used the same thing for years when I was running a plumbing van to carry my shorter pieces of pipe, all thread rod etc. There are lockable caps available for them too.