Rivet nuts are good, but you'd need to dimple the hole back a bit to allow for the flange on the face of the rivet nut so the hinge would fit flush as intended.
If the threads are clean and the bolt screws in easily, gluing the nut on the back side would work, as it just needs to be restricted from turning till it gets tight enough for friction to hold it. Although I think access to that top hole is blocked by the box around the hinge pocket. That points toward the rivet nut solution. The hole size might be an issue for the rivet nut, but not insurmountable, even if it's too large.
I've used rivet nuts to mount fenders to a boat trailer with a box frame side member. Worked well.
Thread size is 3/8-24 like Fox said
Last edited by klhansen; 10/05/2022 8:54 PM. Reason: added thread size
