I use 4.5mm cast acrylic for mine and cut 2mm deep.
I'm not sure what maximum depth you would need to use with 10mm but I would have thought 2 to 3mm would be about right but it is a guess on my behalf.
Here is a video I made of the process if you are interested.
It is basically an edgelit sign with a clock in it.
http://www.youtube.com/watch?v=0Uo6C9TW0_8]Edgelit Clock.avi - YouTube