A quick look as I am busy today shows some cam operation pins but not big enough so I would make 20mm pin for the 12mm pin and bearing fit it on the end.
Probably make the arm it's fitted on adjustable to start with to cope with different gaps near the centre you mentioned.
Using a cam version means you could actually make 2 and leave them in site cammed up when not actually indexing.
The square tube would slide easily along the vise so indexing accuracy would be as good as the stock. In the width.
And as good as the pin/cam fitting on the length.
Probably make the arm it's fitted on adjustable to start with to cope with different gaps near the centre you mentioned.
Using a cam version means you could actually make 2 and leave them in site cammed up when not actually indexing.
The square tube would slide easily along the vise so indexing accuracy would be as good as the stock. In the width.
And as good as the pin/cam fitting on the length.