This may not be relevant, but I'll chuck it in the pot anyway ...
One of the problems I've experienced when building boxes from flat-packs, is that they nearly always develop a twist. I took to using heavy weights when gluing-up, which initially cured the problem ... until the box had rested for a while, when the twist returned.
I've recently bought a glass-topped table, which I'm now using as a reference surface, and in many case I've found that there's a need to open-up the tongue and slot joints with a rasp, until they're sloppy. Then the box is glued-up on top of the glass table, relying on the remaining glue contact in the joints for the initial set. When set, the joints are then given additional glue to fill any remaining gaps.
The twist problem now appears to have been solved.