I recently bought an 18cm x 24cm FKD camera, and when it arrived the lens had not been securely packed; during transit it had smashed the original ground glass into hundreds of pieces.
To be fair to the seller, he is sending me a new one, but in the mean time I thought it was a good opportunity to make my own. My wife has just taken up making stained glass, so she cut some 2mm glass to size and clipped the corners.
Using 600grit Silicon Carbide, it took about 1/2 hour to do, all that's left is to draw the lines on. The glass is really bright and it'll be interesting to compare it against the new one that it on it's way to me!