In theory they are just a sturdy base with a riser somewhat taller than the radius of the Star, which is supported on a good (preferably, set) of bearings with an axle through the center of the Star. the targets can be any size, but within a reasonably tight tolerance of weight (although if you don't much care that your Star is unbalanced, it probably wouldn't matter a great deal structurally) and are held to the arms by a back spring clip pushing the target against a front stop, so that the force of the bullet overcomes the force of the clip and the target falls from its nest (which is usually cut in a pattern to match a pattern on one edge of the target, kind of like a loose fitting jigsaw puzzle). If you have the skills and materials to make one, you would probably be able to figure out a design after just seeing one work. MCFG has a few, and the Bowling Pin match this Saturday will be on the Texas Stars vs pins, so I bet if you showed up with a notebook you could sketch out a workable design in a few minutes. If you don't want to go freestyle, here is a set of plans for a pretty cheap and chintzy one that I wouldn't personally make, but I gotta hand it to this guy; I see absolutely NO reason why this wouldn't work, and it's about as cheap as dirt.
EDIT: Bah, clearly I forgot to link. I'll link it when I get back to my computer, too hard on the iPhone.