Similar irritation with front engine mounting crossmember bolts yesterday, on Skywing/Charade

The bolts are semi-captive (they have a bit of play on them which aids lining things up.)

I dunno how the other ends are retained since they are inside the chassis front box-section but one of them was just going round. I found cleaning the nuts and bolts very thoroughly and putting tension (weight in this case) on the nut would get some turns but it took a long time and was a pain in the neck (literally and figuratively)

Perhaps tension would help in your case.

I thouight of spraying something sticky inside the box section, Maybe spray foam would be stiff enough to have held them.

Or even vegetable oil, (which I spray and paint around a lot anyway) though it takes a while to set.