Looking at the link in the OP, it looks like the base plate (not sure what the proper names of these parts are) with the square hole in it, is replaceable. Assuming that the head is also replaceable, it would seen in the realm of possibility to grind down or replace the head with a smaller size, and have a new base fabricated to fit (the two parts have to fit together precisely, with some small clearance, in order to work properly). I would expect that the thinner the cutter, the lower the the thickness capacity, but if you limit yourself to styrene or similar plastics, it might work ok. Given that for the application, the OP was talking about reducing the head to as small as 1/16 square, you might need to have the head fabricated from a different material if you want to cut into brass or similar, because there would be a lot less material backing up the cutting "notch".