Rivet - (3/16 x 1/2'')

Pop rivets in 3/16 diameter by half an inch. A rivet solves a problem a screw cannot: it fixes from one side only. Set it with a rivet gun, the mandrel pulls a head up behind the material and snaps off, and the joint is made without anyone needing to hold a nut on the far face - which is the whole reason they are used on closed sections, tanks, ducting and box work. The 3/16 diameter is the heavier of the common sizes, and half an inch is the grip length, meaning the total thickness of the material it will clamp. Get that wrong in either direction and the rivet either will not set or sets loose. Drill the hole to suit the rivet, and back it with a washer on soft or thin material.