Spectrum GT - pull-pots question

All:

I have a Spectrum GT that I bought new back in the mid 80s.  It has been through a lot with me, including a lot of gigs, recording work, , a re-fret, re-finishing, etc.  Through all of that I have loved this guitar, but there is one thing that makes me crazy:

I can't get the knobs to stay on the pots when I pull the to coil-tap.  I have tried everything I know.  I'm on the second set of knobs (I got the second set back sometime in the 90s I think), and neither old nor new will stay put.

Am I the only one with this issue?  Anybody got a fix that doesn't do permanent damage?

Cut a piece of old credit card and fit it in the slot of the pot.

Keeps the spindle at the correct size.

Thanks - I'll try that!