This is a good idea. I've just been stuffing empty CD cases in my empty TG boxes. I did make a "form" once to restore an NES Action set box I found in the attic of one of my bosses rental houses. I was helping him clean the place out and couldn't let him throw that box out.. I lightly misted the inside of the box with water then stuffed the factory inner Styrofoam inside and then surrounded the outside with a form fitting box made out of 1/4" particle board. After I let it sit in there for a few weeks it was the correct shape again.
I found about 20 of my old turbo game boxes in my parents attic in squished condition. On most of them the glue and failed so I was able to unfold them completely. I took all the squished ones and ironed them on a low heat setting and then presses them flat for a few weeks in between some boards. I then made a wooden form and reglued them on the form. They came out surprising well.
I don't keep my games in the cardboard boxes and several of my boxes are kinda squished and have seen better days. I had made a wooden form in the past to help re-glue boxes that had come apart, sort of a TG-16 game box shoe tree. I wanted something like that for all of my boxes but not something quite so bulky. I found some rigid styrofoam at a craft store and made these:
I went the more expensive route and bought some TG box protectors - the exoskeleton approach. They're definitely worth it and I don't have as much fear of crushing my boxes, but I definitely dig your DIY approach!