Here are some knobs I made for my Squire Affinity Strat. They are made using Sculptey bake clay that you can find at most art, craft & hobby stores. You bake at 250 for 10 minutes it turns into a solid plastic. I used an old useless pot to mold the knobs around so they would have the pot stem grooves in them. I left the pot stem in when they were baked and only removed it after it was done. It's best to use an old pot, I'm not sure if 250 degrees would ruin a good one but it's best to be safe. They work perfectly, and the knob shapes you could make are endless.
The clay even comes in a bendable rubbery version called Bend-N-Bake that may give a better feel or "grip". You could use this tip to make knobs for effects too. This clay is also good for prototyping other small plastic mechanical parts and do-dads, it is pretty strong stuff but there is a limit on the amount of strain it can endure.