I want to grind the three 14" mirrors in parallel and expected to grind through several layers of tiles in the process but found a neat idea at the Mirror-o-Matic group on Yahoo, I turned two-inch tiles (about three 12x12" sheets) into one-inch ones with a tile cutter, taped them into strips and coiled those on edge to fill the form. At about the 4 o'clock position you can see where I started the spiral.
Lay ten tiles flat in a row with about 1/8" between them, glazed side up. Use a straight edge to align them on one side and apply a strip of tape. I used sheetrock joint tape cut into one-inch wide strips, but masking tape would probably work, it is just there to keep the tiles from falling over (but the fibers in the joint tape might make a stronger tool). I mixed enough plaster to cover them about 1/2 inch and jiggled and poked that down into the cracks. Then mixed and added another inch of plaster. After an hour I removed it from the form, flipped it over and filled in the cracks with a little more plaster. When that dried I scraped the covering plaster off the tiles with a putty knife. It is ten inches across, two high, and weighs about nine pounds. Hopefully it will grind all three mirrors.
Dental plaster goes for over $30 for 25lb around here from dental supply places. But I found a lumber supply place that sells 100lb for the same price. Doesn't hurt to call around.
There is a good description of setting up and pouring a spiral-wound tool at: http://www.atmsite.org/contrib/MurrayJohn/mom4.htm . John had problems with the outer tiles coming off, but I suspect putting the tape side out will cure that, he also used a radial pattern (click Next on his page). On another tool I put a wide strip cut from fiberglass window screen around the inside of the form before stacking the tiles and pouring the plaster. That will hold everything together very well.
There is a pic of an different edge-stacked tool at: http://www.mtburr.com/binoscopes/tiletool.htm