Mirror making classes in Eugene Oregon for years have featured John Walley's 7 inch diameter 3/8 inch thick glass cut from bank doors. You will want to make a f/10 from that 4 inch 1/4 inch glass since you have no room for much depth or sagitta in the center of the mirror. Also you will want to be careful and use the standard techniques to reduce astigmatism. Mel Bartels