Many thanks to Sam Duwe (WN5C) who writes:

John (AE5X) had suggested a while back finding a larger tuning knob for the Penntek TR-35.

I was striking out in finding one that fit but had to put in a Mouser order and thought I’d give this knob a shot (Mouser # 706-11K5013-KFNB) and I really like it!

I accidentally bought the gray version and had to paint it black. It’s shiny because of the clear coat and rough because of sanding and because I’m apparently awful at painting, but I find that it fits with the plastic bezel of the radio.

With the optical encoder it really glides, and totally changes the feel of the radio. Pretty good deal for $2.36.

Thank you for the tip, Sam!