This is the way I have been doing it for years. I use frosted mylar as it transfers no texture onto the print. I don't cut the mylar at all. Instead, areas that will not receive any flashing are painted in with red paint directly on the mylar. I also dodge/burn this way. To diffuse the light from the enlarger even more, I place a think piece of white plexi under the lens. Most effective.Why not just take a sheet of opaque (or even semi-translucent) paper same size as printing paper, place in easel, and sketch areas to be cut out for pre-flashing. After cutting, then place that mask over the paper in the easel & pre-flash? Would seem to give you most of the control as above but without an intermediate development step.