I use band or hack saw (depending on thickness of riser) to make relief cuts, and then cut out the shape of the window, stopping short of the line. I use the dremel to clean it up, then sand. I found that the grout removing bit, has teeth like a rasp, and works like a charm. I then draw the arc of the shelf radius and shape that with the dremel too. some folks use a disc sander and rotate the bow around it, but when i bought my combo belt/disc sander i didn't buy a model where the disc cleared the motor housing (don't do that if you are shopping for one). I not only radius the shelf, but i angle the shelf slightly, higher to the outside and lower to the inside. this helps keep the arrow on the rest.