I've always struggled with the rubber bands I use to hold jewelry box sides square and steady during glue-up. The biggest pain in the neck was cleaning the glue squeeze-out from under the bands. When I lifted the band the box would shift before my eyes! I was ready to chuck the whole mess in the garbage when I came up with a clever new way to hold the boards. I made L-shaped corner blocks with stiff rubber bands in between. The inside corners are cut back to prevent glue squeeze-out from gluing the blocks to the sides of the box. Now glue-up is a joy. I apply glue to all the mitered faces and slide the long sides inside the blocks. Then I slide in the short sides. The inside diagonals are measured and the box is shifted until they are equal. The rubber bands can be replaced with longer or shorter ones for different sized boxees.
I would take this one step further and cover the inside faces of the blocks with packing tape to avoid Murphy's Law.