Hi,
Yes the cutoff wheels work pretty well. They make at least two different sizes. The big ones are fiber reinforced but the small ones are not. The big ones make a slightly wider cut however, while the smaller ones more narrow. The smaller ones also wear out faster than the big ones. Either way these are probably the best bet.
They do make steel cutters like little saw blades on a permanent shaft, and they actually make little saw blades for mounting on a mandrel too but they might be hard to find. The steel cutters wear out too fast though when cutting through fiberglass board, although they do still cut after that. Some of the little saw blades they make have teeth that are far too big and get caught on the work piece so it's not a good idea. The small teeth saw blades would be ideal, but they would probably dull too fast, and also i dont know where to get them anymore.