I make my own and the key is getting even pressure on the grinding glass, the smaller piece. The larger piece of glass you use to grind with, the less even the pressure becomes. A smaller piece allows you to apply more even and consistent pressure while you grind, at least that is my experience. Also, as noted, while you grind the slurry begins to dry and the glass will begin to stick so you want to be able to remove it easily. All in all, it is not a difficult thing to make.

Also, I keep frosted Scotch tape in my repair box in case I crack a ground glass, it works well in a pinch. I was out yesterday with a friend and his 8X10 blew over and his glass shattered, not much we could do with that except head home.