This is a cheap and easy to make knob, using just an ordinary plastic soft drink bottle top, a bolt, a nut and two washers.
It is especially suitable for projects where a bolt needs to be turned to adjust something.
I'm actually surprised that a similar instructable has not yet been published, as it seems such an obvious idea. My sincerest apologies if it has indeed previously been published and I missed it.
Step 1: The Parts
The picture shows all the parts necessary. Actually bigger washers would be better for a tighter grip on the bottle top.
Step 2: Preparing the Bottle Top
Just drill a hole in the centre of the bottle top the size of the bolt you're going to use.
Step 3: Assembly
The first picture shows the order in which the parts fit together. Then just tighten the nut very tightly. A lock nut is preferable, and you can of course also add glue to help avoid slippage. I usually use an ordinary nut, and lock it by damaging the bolt's thread beforehand with a metal punch (second picture).
Step 4: Finished
The first picture shows the finished knob. You can of course also use different size tops (second picture) for the appropriate torque/control you need.