By avoiding the need to print the spherical balls necessary for a ball bearing, I my design is nearly as small as a stock 608 bearing.

This is still a proof of concept, and not meant to immediately work. The little plastic tab does not adequately fit into the slot on the outer ring. In a further redesign I would shrink both rings to exactly match the dimensions of a 608 bearing and move the hole for filling the bearing to the inner ring.

For the spherical balls inside I tested the small plastic pellets from craft stores used to stuff knitted/crocheted objects. As long as I chose the more spherical ones, these were reasonably acceptable. Unfortunately, as these pellets were not very uniform I spent too much time picking out the best bits.

However, I found a much greater success with using small 3mm spherical beads from a craft store. These were all essentially identical in size and shape. All of the parts have a hole through the center, for them to be used as beads, but this feature did not appear to impact the functioning of the ball bearing.