Knobs
In this project, we used T18 knurled shaft potentiometers and corresponding 6mm shaft T18 knobs. These were chosen because they don't require any extra tools, like Allen wrenches or tiny screwdrivers. Instead, you can simply push down to install the knob.
For alignment: First twist the shaft of each potentiometer to the minimum or maximum setting. This gives you a reference when choosing the knob position. Next, push the knob on.
If the knob feels too loose, pull it back off and carefully and gently pry the two halves of the shaft slightly apart, just a hair at a time. Moving it too far can snap it off.
Customization Option
Another easy way to customize your pedal is with different knobs. Look for ones that fit T18, 6mm, knurled shaft potentiometers. Here are some from LoveMySwitches.
If you want to use 6.35mm (1/4") solid shaft knobs, you can buy adapter sleeves to fit around the T18 knurled shaft.