Fixtures for boxes should be relatively easy, and reasonably universal.

The hardest part will be coming up with some form of clamping strategy to give good support. If it was me and I was planning on doing lots, I'd be looking at some form of large open vice like setup, which allows the box to be stood on end/edge, and clamped on what would be the top/bottom (or front/back depending on if you class the lid as top or front).
Add in an end stop for locating the box against prior to tightening, and you've got a repeatable clamping setup.

For doing the clamping, some form of toggle clamp setup would allow for quick changes, but would need someway of being adjusted for different box sizes. If you really wanted quick changes, an air cylinder and valve would make things quicker, and cover a larger range of sizes without adjustment.