Here is a quick and actually good mod i did for the radio shack dx-394 VFO knob. I used 4 hex nuts one for each slot inside the vfo and some hot glue. That made a huge difference in tuning. Now it does not feel so toyish.Go easy with the hot glue.
If you make a mess just use a lighter and go over it a couple of times (not to much or you will melt the shaft hole)
Here is the info on the nuts i used.
Item # 6212, 1/4-20 Hex Nut on The Hillman Group Total cost was $1.19