This is a take up slack collar that Joe Person, a very smart builder put on FB. Seems like a good idea although it has taken a bit to get my head around it.

I made two, the first seemed to have more adjustment than needed so I had another go. Once the construction method was clear they are really very easy to make.

Two holes in the right place then just join them up with drilling and then the dremel. All of a sudden we have pic 3.

Drilling out end points

File and drill curves

Simple eh?