Here are photos of another painting jig. These are from a video by Ron Marsh of Ron's Trains N Things.
The jig consists of a block of wood and two bamboo skewer segments. The skewers are placed in holes drilled to the same spacing of the bolster screw holes in the model to be painted. The block of wood needs to be fairly wide to provide stability when spaying paint from an airbrush or rattle can.
Be careful about where you store this jig as you don't want to accidently puncture your hand if the points are exposed.
Here is a link to the source video, which is about weathering tank cars: