Back wall of the frame or back wall of the building?
I would not advise using hot glue - it will NEVER come off. Position each car where it will be in the final presentation the poke a hole through the backing material (I use a hammer and nail) under each tire. Using a needle and 5 pound test run the thread up through the hole, around the axel, and down through the same hole. Repeat for each wheel of each car.
Inexpensive? No. Reversible? Yes.
Have you considered mounting some of the cars on the side walls of the shadow box? Just a thought. Kit
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Time flies like an arrow; fruit flies like a banana