Trimpots come in a variety of shapes and sizes. Most often, they have slots suitable for screwdrivers. However, a few trimpots have control knobs that are intended to be turned with fingers.
Trimpots are fairly expensive. So, you don’t really want to double the size of your collection by stocking both screwdriver and knob based potentiometers. That’s part of what motivated me to 3D print knobs to attach to normal trimpots.
Left: Bourns with official thumb dial (3386F-T) Right: Bourns 3386W with red printed dial
The other reason that I wanted to make my own potentiometer dials is so that I could have a variety of colors to indicate the purpose of the pot, or at least to better differentiate between them. (“Turn the red knob to release the power!”).
I modeled the trimpot dial in Tinkercad.
Bourns 3386 knob 3D model
In hindsight, I wish I had also taken the time to emboss an arrow or slot in the face of the thumb dial. This brings up another advantage of home-printed dials: you can customize them with letters or symbols to indicate their purpose.
The little rectangle in the dial needs to fit snugly in the screwdriver slot. I needed to use a metal file to reduce and square up the edges of the raised rectangle after printing, in order to get a good fit.
Screwdriver slot in trimpot and mating rectangle in dial
After testing the fit, clean both surfaces with isopropyl alcohol. I know you want to skip this step, but unless you reduce the oils on the plastic surfaces, the thumb dial will not stay attached.
Finally, apply a tiny bit of adhesive and press the parts together. Don’t use too much, or the excess adhesive will prevent the dial from turning.
3M plastic emblem and trim adhesive 03601
I used 3M-brand Plastic Emblem and Trim Adhesive (part #03601). It received mixed reviews on Amazon, but it works surprisingly well in this application.