i just used a nut about the same size. weld a 1/4 inch bar on the side of the nut 2 or 3 inch long. fish it in with a wire over the hole and thread your bolt in. the bar keeps it from turning in the frame. 
Cheap and effective. I like it
The only concern I see is if I need to remove the crossmember, and then reinstall it, and I loose the nuts in the frame. I guess that's what magnets and mechanics wire is for. Also, if the bar breaks off while removing the bolt, I'd have to resort to cutting the bolts out.
