I have 15" wheels from a 1/2 ton G10 van. These wheels have the same bolt pattern as my Camaro rear end and front disk brake rotors. (5 bolts on 4 3/4" BC). The wheels have the bumps for "poverty" style caps. This is how I made my reproduction 1954 caps work.
First, I picked up some caps from the local auto wreckers. These caps were very common in the 1970s era and match the bumps on my wheels. I cut these up making tabs so I could pop rivet the reproduction caps onto them. I think the attached pictures are self-explanatory.