Defiantly want to use either RTV 108 or RTV 103 from Grainger. As far as glass thickness, there is a chart floating around on the interwebs with how thick you need to go depending on what distance you're trying to cover.
You would prob need at least 5/8 glass or better. I like the 30" from front to back but it kills you cost wise cause u need the thicker glass. When its done it will b nice to scape though.