I just tried using the suggestion of trimming your boxes to width after being built so they'll be "perfectly even".
Just a bit of clarification re: trimming boxes accurately to size.
These are the very last National Brood Boxes I made from pallet wood, which was before Mann-Lake came to town, and was the only viable way from me to acquire cheap boxes.
Now I buy Mann-Lake boxes flat-pack, and although being 2nd-grade they need some fettling, it's more cost-effective than breaking pallets apart for their wood.
Ok - so having run the planks individually across the table router to clean-up their edges, I then glued-up wide composite planks to form the box sides and ends. These are run across a table saw to reduce them to size, plus a few millimetres extra for trimming. Then the boxes themselves are built. In the following shot, the end plates were obviously left very proud (can't think why, now) but would then have been cut down to remove most of the excess, but leaving enough for the next step.
The boxes are then placed on a dead-flat bed and checked for rock - which I find is a constant issue when working with pallet wood. If it is more than just a couple of millmetres (which would be ignored, as subsequent loading weight would flatten that) then spacers - bits of old credit cards, washers etc - are inserted equally at opposite corners to stop the rock, and then hot-glued in place.
Then the sides of the jig are built up, and a router sledge placed over the top. This is run back and forth until the edges are uniformly flat, at which point the jig is dismantled, and the box secured with the flat side downwards (no need for spacers now) and the process repeated on the other edge until the desired box height is reached.
Because this router is fixed to the sledge and thus dedicated to this one task - likewise the router fixed under the router table - I've used cheap and cheerful routers (ex bring and buy sales) for these tasks. But to make such cheap and simple plunge routers more useful, I've fitted them with a means of precise adjustment:
This very simple modification makes all the difference in the world, turning a fairly crude and simple plunge router into a precision tool with controlled variable depth.
LJ