This is a Gimbal designed for a DIY SteadiCam that I made for a colleague at work (See http://www.yb2normal.com/DIYsteadicam.html).
It makes use of two 624 bearings in addition to the required 608 bearing.
It would also be possible to use four 624 bearings or to use no 624 bearings. I thought using two was a good compromise. If you decide to use no 624 bearings then the height of the middle ring can be reduced to 16mm (same as the outer ring) or even 12mm.
First glue the 608 bearing to the center of the Inner ring and the two 624 bearings to their proper location in the Middle ring. Then attach the 4mm cap screw from the outside of the Outer ring to the Middle ring. On each side of a ring there should be a washer. It should be as follows: cap head, washer, outer ring, washer, nut, nut, washer, 624 bearing, washer and finally the nut.
Now attach the Middle ring to the Inner ring by using 4mm cap screw, washer, middle ring, washer, nut.
Adjust as necessary to get good movement.