This is a reduction drive for the little plastic variable capacitors. It uses an 8mm bolt and an o-ring belt. Approximately a 5:1 ratio. It even has space to put labels.

It should fit most of those little plastic variable capacitors. All the ones I've found had the same shaft size and shape. You'll need some short 2.5mm bolts for the capacitor. If they're too long they'll short out the capacitor.

The knob was designed to fit my specific SHCS, so it may not fit yours. It has a little 6mm hexagon inside to lock it into the bolt head.

I uploaded both a bottom mount and a panel mount version. I may post a version for air variable capacitors when I get some.