If you don't mind the plate overhanging the sides Swill could add sides with notches to clip into the top and bottom plates quite easily, I wonder if he's thought about it.
If you can show me what you mean I can probably add it.
It's really easy in my head but how to explain...
You'd need to add a "plate thickness" option then make two of each of these shapes for the ends and sides, where the steps/slots are [thickness] so it fits flush.
![Custom Case Making Tools? 99588-0](https://geekhack.org/index.php?PHPSESSID=sho6t48aforqie6ob13t84no72buu4d3&action=dlattach;topic=71578.0;attach=99589;image)
I put the slots on the front/back so it makes a nice flat face, but I'm not sure if slotting the sides together is needed as it's not very tall.
The downside is that the top and bottom plate will have to have an overhang to clip into but as it's unlikely there will be any outward force and you can't bend it inwards while in use as there would be a notch in the slot maybe 1mm would be enough? Depends how much you trust your postman!