I've been looking for a solution for my speedometer relocation. I needed to find a way secure my trim ring that will hold the indicator lights. I was looking for some sort of o-ring, gasket, or something with a 3 3/8" internal diameter. I went to the plumbing department of a local hardware store and found this thing.
I don't know what it's called but it looks like a way for connecting two pieces of pipe. It's a big rubber ring surrounded by corrugated metal and two worm clamps.
The part was marked 3" but the inside diameter of the rubber ring was a perfect 3 3/8". I'll never understand plumbing measurements as I had a similar issue when buying a PVC cap for my ignition switch relocation.
I was able to cut the rubber ring into strips that should work perfectly. They fit nice and snug around the speedometer. More on that later.