This screed is valid for 1/4" shafts...
Hardware stores will carry a 1/4" rotary file that should do a good job incrementally, without the potential grabbing and cracking of a drill. Or the out-of-round situation of using a Dremel to nibble away at the hole.
Here's a link to one:
https://www.stinehome.com/forney-rot...d-plastic.html
( If you have to go the other way, try these:
https://www.ebay.com/i/323825291229?chn=ps )
Now, how to actually do it...
If you can hold it in a vise without scratching it all up, do that.
If you can't, get one of those kevlar kitchen gloves that prevent you from getting cut. Then, hold the knob in one hand and your varispeed drill in the other, and keep it slow and straight. Verify alignment often, and make sure it's cutting and making chips rather than just heating stuff up. The file doesn't have grabby teeth, but still make sure your glove doesn't get all wrapped up.
You could even do the 'no power' option and just twist the knob onto the file as if it were the shaft. Safer but slower.
A hand reamer would also do the trick. Sharper, more precise sizing if you need, non-powered, and more expensive. But it still beats filing away on a perfectly good volume control shaft, and having to deal with concentricity, filing debris, and danger of scratching faceplates.