I finalized the idler plans, measured the height of the carriage motor at the belt location and used my idler parts thickness to decide the full height of the idler assembly. A quick cut out with the laser cutter and an overnight glue job yielded a final part. Lastly, I took a picture of (almost) all the coiler parts together. Not appearing in the picture are the electronics and the magnet wire spool holder assembly.
The CAD for the idler parts is not the final version, I dropped the curved top but you get the idea.