Williams
Jun 20,2023"cut a slit in 12mm MDF" Rather than vaporize all that MDF, slice the board with a saw, then glue the two pieces together with a strip of of 3 mm MDF standing on edge at the bottom to form the slot. Cut the strip to whatever width makes the slot the proper depth. This depends on being able to saw-cut straight paths, a trick I generally got right using a radial-arm saw, Doing it by hand poses a definite challenge.