I still don't understand why most of ATMs start from flat blank/tool (especially if very deep curve is needed, like f/5 and faster). If you can't get pregenerated blank, you can easily "pregenerate" it yourself. Just use small tool (I use a steel ring from an old ball bearing) and with mirror face-up hog out desired depth. Another advantage is that you don't loose any thickness at the end of a blank. After getting rough curve on it, cast the plaster using now concave blank, and glue the tiles. Presto - convex tool of roughly desired radius, and ditto for concave mirror ! Only short grinding with #80 will smooth the tiles, and at the same time get mirror and tool in good contact, ready to continue with next grit. Make sure that blank is well beveled, and use LOTS of pressure with ring tool. You'll be surprised how fast it will cut the glass !
Bratislav