First, locate a ceramics supply wholesale store and buy from them a quantity of industrial strength waterproof plaster. They use this as molds for firing greenware or something like that. I picked up a 100 lb bag of this stuff a couple of years ago for $12.00.
Make a collar for your mirror just like you would make for the tool when pouring pitch for a lap. In this case though, you will want the collar to be at least 1.5 to 2" above the surface of the mirror.
Place the mirror face up on a level surface and place the collar around it. Use a very small quantity of dishwashing liquid as a release agent on the mirror and the inside of the collar. Mix the plaster to the consistancy of pancake syrup and pour it in the mold.
Pour it slow to avoid creating bubbles, but other than that, you are finished...or will be after it sets overnight.
You then can pour your pitch lap and treat the tool as if it were pyrex.
We go a step further when making these tools for grinding. We use ceramic floor tiles (either 1" or 2" work fine). Willman Bell sells hexagonal tiles for the same purpose, but I haven't seen that the fancy shape of W.B.'s really makes any difference when compared to the results of the square ones.
There are two ways of "mounting" the tiles and both work well. The easiest is to just lay the tiles directly on the surface of the glass that will become the mirror and pour the plaster over them. You have to be careful with this though to insure that the tiles do not get moved out of position (they should be separated by a space around a quarter inch on all sides.) These tiles form the grinding surface.
The other method, and the one I prefer, requires pouring the plaster first and letting it cure. Once it has completely cured, Epoxy the tiles directly to its surface. Again, separate the tiles so as to form 1/4" channels.
With the embedded tile-tool, use it just as if it were a glass tool.
With the channeled tool, the procedure is a bit different. Charge the heck out of the tool...filling the channels with the heavier grits! Your grinding will go far faster than with the more conventional tools because you don't have to continually clean and charge them. Granted, you will probably wear out before the charge is exhausted, but you will be able to go for 10 or 20 minutes before having to recharge.
The finer grits are no different than with a normal tool.
Hope this gives you some ideas.
Mike
* RM 1.3 * Eval Day 6 * Diplomacy: Saying "Nice doggie" until you can find a rock