LOL.. I will have to get a picture of mine.
I started with boards squared and nailed to the table where I built the boxes, but those boards were always in my way, so I now have a 1 inch thick piece of particle board with the jig nailed to it. I can hang it on the wall and get it out of my way, then take it down, set it on the table and go.
It is basically just boards that my box pieces drop into. I glue, drop in and nail. It would not work with finger joints, but I use rabbits on the end boards so they slide into place nicely and are held there while I nail or staple the box together. Lift the whole thing out and stack the boxes as I go.. I usually do ten per stack and then weight them.
I tend to rush when cutting a lot of boards so occasionally have a few that do not set flat, but they are usually quite square.
I will get a pic of it next time I am at the shop.