It took a couple tries, but it looks like I came up with a working idler post and race. As with all the other fittings for 608 bearings in this project I’m using crush ribs for a secure fit, and using the insertion tool I designed to push the bearing into the hole in the pulley putting pressure only on the outer race of the bearing.
I’m drying a new roll of filament to print the box. This roll is nearly done.