I’d shop around at your local glass shops. You could even have them use scraps, recycling glass, since a few minor scratches won’t matter. To get the dimension, I’d cut a piece of plywood close to the thickness of the glass you are going to use. I had a piece cut slightly too big, not thinking, I’d have to turn it and the corner to corner dimension wouldn’t make it. BTW My custom top was right in line with what a store bought top would have cost. Don’t forget to have them smooth the edges. Not only will it prevent you from getting cut, it prevents a place for a stress crack to start.