M.O.
We have "Michael's" craft stores by us. They carry shadow boxes in all sizes. Wood frames with glass fronts, and black felt on the back. I would go and see the sizes to see what you can fit in one. That will help you determine WHAT can go into the box. It sounds like you have some big items/a lot you want in there.
I agree with a pp that to "mount" the cars and hacky sacs hot glue might work, but perhaps using fishing line to sew them onto the back so they stay supported. Push pins might work, but they may give way too over time.
I don't know how you're going to make a "boy" box country looking tho?
Best wishes, sorry I don't have more input.