A tool we have around here that i THINK
would be good for modding if something called a RotoZip. I have used one to cut ceramic tile for my kitchen, plywood etc. Comes with several different types of bits for different materials. Jerrygarcia uses one fairly often in his electricians work. His has an attachment like a compass sort of that lets you cut a perfect circle. Also has a right angled wheel attachment for sanding , grinding etc.Let me find a link for you. A UK link:
RotoZip
Look in Rebel accessories , check out the circle cutter , cuts 2 cm -51 cm diameter circles. Plus for ME, I could use this in lots of other applications around the house.