You are right, I forgot about the gap caused by the long track on the focuser tube...
If I may, I would suggest you stay away from electrical tape because, after some time, it will leave sticky glue on the contact surface. I use masking tape to lock the IS switch on my 100-300mm lens and it performs well without leaving any residue.
Personnaly, I'm not too worried about small dust particules on a lens because they can't be seen on the photo most of the time. Also, they are quite easy to remove if they bother me. Same for the scope, both inside out.