I have had trouble with the tuning knob on my Elad FDM-DUO ever since I got it. The machining of the threaded hole for the set screw was poorly made. You cannot order a new one from Elad because they are always out of stock. Finally I found a very good replacement from LoveMySwitches.com. The set screws engage the shaft very well even though the knob info says it is designed for use with a "smooth" shaft. The O.D. is very slightly less than the Elad knob. Of course the encoder is still a bit wobbly and the new knob has no effect on that.

Silver Anodized Aluminum Knob - "Full Moon" - 1/4" Smooth Shaft (41mm OD) $7.99

https://lovemyswitches.com/silver-anodized-aluminum-knob-full-moon-1-4-smooth-shaft-41mm-od/