Have you called a glass supplier? That's where I get them. Kitchen and bath showrooms will have resources, too, but a glass supplier is usually a bit less expensive and they do this all the time!

I second that- a glass supplier. They should come out, measure, create a template and cut it custom for you. Good luck!

I have done this. I sourced a piece of housing from the shower screen supplier. It ran Vertically down the tile to the height of glass. Nothing on the bottom - only the silicon on each side. Looks fantastic and is soooo easy to clean.