Just to update my post. As I was discussing whether to do this or to just pull the trigger on a custom felt with a betting line, my players surprisingly agreed that a betting line was totally unnecessary. Okay then.Nice work. The plywood stencil sounds safe.
I’m still pretty sure you could just rig up a marker to the end of a 10 inch stick, put the other end against the rail, and just push it all the way around the table, and end up with a perfect betting line.
I haven’t had the guts to try it yet, but I’m not sure why not, because you could easily practice with chalk.