You can also take a regular flat slate (or a piece of scrap plastic) that is close to the size you want and boil it for a few minutes. Once it's good and hot, fish it out of the water with some tongs and bend it around a drinking glass (a pint glass 'liberated' from the local pub works good for this
). Let it cool and there you have it. You can drill the holes before or after the bending, but before is easier.
Oh-and if you use scrap plastic instead of a 'real' dive slate, rough it up with some fine grit sandpaper first.
Hope that's helpful.
Brian