Duh, I've been running that router so long, that I momentarily forgot what I cut my teeth on, I have used my Porter Cable oscillating sabre saw like forever. Yeah that would work.
But a sample just arrived, about an hour ago, of some new "U-blast" high tack sandblast stencil and It's very thin and conformable. I just tried it on a piece of bare concrete block and it sticks very well. Well enough to use as a mask. I think we will be going that route. Thanks!