What hardware to mount to bed rail

I want to mount some angle iron to the bed rails of my truck, and I was wondering what nut and bolt combination other people have used to do this task.
I tried to use spring nuts, the kind we use with unistrut, but they are too big and do not turn and lock into place.

 

I've always used uni struts on mine, bestop sells a soft top bed cover for our trucks and they come with 4 aluminum clamps that have a t shaped nut that slides perfect in the rail and it uses a 3/8 thread you might be able to order a few from their customer service department.