It sounds like you have a good thickness of paint built up in the trench. If having to be forced to create rectangular openings in the trench, i.e. being asked by a client to do so, I would try the following ;
Create a sheet metal guide. Very thin brass, .020 or so. Drill and shape about 6 to 10 rectangles, or more, at different levels, i.e decks. The height of the brass guide should be much shorter than the trench is. This will allow you to raise and lower the guide to vary the positions of the hangars. To keep the openings level, use different thicknesses of styrene strips to rest the guide on. Pre-bend the styrene strips to lay flat against the rear trench wall. Make sure you clean up the back side of the plate of any burrs so as not the scratch the primer as you move the plate down to the next position.
Using an old x-acto blade, grind it down to fit into the rectangles in the plate. Scrape away..... Tedious, but easier than drilling, IMHO.
Experiment on a primed pic of acrylic, should work. These scraped openings will not be as bright as the drilled holes, but I cannot think of another way to remove the primer precisely and in an organized pattern.