I tried to make the scad file pretty easy to configure. All the configurable items are at the top in sections.
Everything is in mm.
The size parameters are fairly self explanatory, but I should point out that a minimum outside diameter is calculated and if it is larger than the one you set yours will be ignored.
Probably the most important size parameter is ball_spacing_ratio which gives the thing enough room to spin freely. I tried a few and found that 1 is too tight, 1.2 is too loose, and 1.16 is just about right for my printer.
The shape parameters have comments to describe them, but it can do quite a few things. Experiment to find it out!
The bearing.stl file has a 19mm opening and a 39mm outer diameter with a 4mm flange and a ball fill hole. It takes 6mm airsoft bbs as balls. Also, a 1/4-20 setscrew pretty much plugs the fill hole perfectly.