I think they are for outdoor electrical mountings, or applications, actually. They are called "Blank Round Plate Cover" or something like that and are available in 4" diameter; I think they were in the outdoor electrical section of the store. I got them at Orchard along with the clamping knobs. They are about $2.35 apiece or something like that. What's nice is that they come with the weatherstripping seal. I used Gorilla Glue because it would glue foam to painted metal. I bought two titanium nitride plated drills, a small one to start the hole, and a larger one that would fit the clamp knob "bolt"...it's a 1/4 by 3/4 or something like that. I used a punch to mark the center, then drilled a smaller hole first, and then the larger hole after. The steel is pretty tough, so you need good, sharp drill bits. Also, when you do the gluing, just use a thin strip of Gorilla Glue in a circle, concentric with the center of the weatherstrip area, it will tend to squeeze out the sides if you use too much. You can easily clean off any excuse glue that has not set up with isopropyl alcohol (rubbing alcohol) wetted on a paper towel if it oozes out.