I've now printed (and designed) several fidget spinners which use the 608 skateboard bearings. I've got a bench vise, so installing them is pretty easy, but I managed to mash one up using a socket when I tried to press it out, so I designed this really simple bearing press specifically for use with fidget spinners.

There are two versions of the two major parts, one designed for use with an 8mm bolt which fits through the center of the bearing, and another designed for use with a vise. The bolt version has holes in the center, where the vise version is solid in the center.

For the bolt version, I used a M8 bolt that I had on hand which is 30mm long - if you're buying at the hardware store, I'd go at least 35mm as the 30mm was pretty short. Update: I've added a printable M8 bolt and nut from the "Nut Job" customizer. Works well with the bolt version design and makes it fully printable. Considering making one of the pieces (probably the cup) with threads in it to eliminate the need for the nut, but haven't done that yet.

Update: Check out webdad's make - he used a 5/16" SAE bolt instead of the M8 bolt (almost the same exact outer diameter) and printed up knobs for the nut and bolt: http://www.thingiverse.com/make:275497