can somebody tell me a good way to cut a port hole? i normally get them pre-cut, but i wanna switch out with ones i already have that don't have one. in other words, im a noob to cutting the port hole, because ive never done it. i just don't want 2 mess up the head, or make the hole uneven, wear it may eventually tear. maybe just a razor blade, but how would u get that "perfect circle"? and also will it affect the sound if you don't do it right?