I am using a Dremel with plunge router attachment to route a round groove in both my upper and lower MDF boards. The grooves will serve als races. Thanks to the crappiness of the Dremel router attachment, one of the grooves became a spiral. But since I will cover the turntable with something pretty anyway, this won't be a problem.

I then filled the bearing with 8 mm steel palls (8 € for 100 pcs on ebay). The groove was just wide enough to hold the balls in place but I decided to ad a safety cage to keep them from falling out just in case. The cage is made from 3 mm plywood and was cut out on a scroll saw.