You must ensure that the end of the piston ring is absolutely square to the way the piston ring sits in the bore, and there must be no burr on the edge. There is no way you are going to do that with sufficient accuracy with a normal file. Pinching the ends on opposing sides of a spinning dremel cut-off wheel is an interesting idea but I would consider it to be a risky one. If the end of the piston ring is not absolutely square, the remaining opening between the two ends of the piston ring will be bigger than it needs to be, and it doesn't take much for this to lead to excessive blow-by.
No, I don't have the right tool for this. I've had good luck with not needing one so far (end gaps checked out OK straight out of the box).