So Ross suggested a CAM lock that was better than what I was originally thinking. If the ABS flexing is not problematic I think it will work. I am less happy with the spring loaded spool. I wanted it to help provide tension in the system – but I think it looks like overkill.
Hmmm, I think I am going to add the mount points and start building. Just build the carriage last after I have thought about it some more.