I've seen one or two passing mentions of using bottle caps for volume/tone knobs, but no details. It's not that difficult, so maybe everyone else figures it out just like I did.

But in case you want a head start, here are the specifics:

Materials: a 3/4" wood dowel, a bottle cap, glues (silicone or ceramic, and epoxy)

Dimensions: cut a piece of dowel 3/8" long. Drill a 7/32" hole in the center, all the way through. If possible, use a power saw to keep the sawed surfaces parallel and square to the sides and use a drill press with a vise to get the hole parallel to the dowel. This will make the knob fit straight on the pot shaft and turn concentric with the shaft (no wobbling).

Assembly: glue the dowel to the inside of the bottle cap with silicone caulk, ceramics glue (e.g. Duco) or whatever. With modern bottle caps, there is a plastic seal inside; the dowel will fit just inside the sealing ring, and the glue will only hold it against very light force. The only thing the glue does is hold the dowel in place and keep the epoxy from seeping into the center hole. Mix up about a teaspoon or two of epoxy and push it onto the gap between cap and dowel with a toothpick or coffee stirrer, making sure it's more or less evenly distributed all around. Let it harden overnight.

A 7/32" hole is slightly smaller than a 1/4" pot shaft, so you'll have to carefully squeeze it onto the shaft the first time. The metal of the shaft should press grooves on the inside of the wood, which will make it go on and off easier the next time, but still provide a snug fit with no slippage.

There you go - awesome knobs for about ten cents in wood and epoxy.

Metric users: 3/4" ~ 19mm, 3/8" ~ 9.5mm, 7/32" ~5.5mm, 1/4" = 6.35mm, 1 tsp = 5cc.