We use lag bolts to install the box with, the nut and bolt system does not work for the trailers we sell. Just drill a hole in the toy box and trailer, take a ratchet and run the lags bolts in, make sure not to over tighten them and strip them out. I have been wanting to drill the trailer cross bar, run some threads in it and see if the cross bar itself could act ad a nut, all you would need is a good tap set and I think it would work w/o problem.
What type of trailers do you sell?
Taking the wall thickness of the crossbar into account, I'd be very concerned that lag bolts will eventually loosen. Use bolts long enough to go through the crossbar and finish it off with nylock nuts. Use fender washers inside the box so the bolt doesn't pull through the plastic.