You could use something like this.
The grey cylinder is a sprocket, and the side plates (blue) bolt on to the KOP pneumatics mounting brackets (green). In our case, we were going to attach a spring of the right tension to the open axle (bottom-left) and to a bolt which would be driven into the top of the mounting block (red) between the Mounting brackets; but you could as easily attach a pneumatic piston, or anything else.
Scale reference: the red mounting block is meant to fit around 1" square tubing.
