Since I'm relocating my ignition switch, I want to cover the back of it. I picked up a PVC cap. I thought I needed a 2" cap because the diameter of the ignition switch is just under 2" but the 2" cap looked huge at the hardware store. Since they're cheap, I picked up a 1 1/2" cap too just in case as it looked to be closer.
I'm glad I did. The 2" cap was huge and the 1 1/2" cap fit nearly perfectly. Here you can see the cap slipped over the front of the switch just to illustrate the fit. Plumbers and pipefitters can probably explain why the 1 1/2" cap fit even though it's not 1 1/2" diameter but all I care about is that it works. From this photo, you can also see the back of the switch and why I want to cover it.
I carefully cut a notch using my Dremel and you can see how it slips over the back.
Once I was happy with the fit, I sanded off the logo and text on the cap and roughed up the surface for paint.