Long walls warp/bend

[Alex9779]

Came up to my prints of inserts for a 5l box with two layers and a grid of 1x3, the long side walls bend a little. They are still working find and stable, but maybe it would be possible to design some little "stabalizers" like just the very least of a perpendicular wall start in the middle or at configurable distances, just 1-5mm long. That should be able to prevent that problem...