Contact this GG maker. He has a direct eBay store.

He's made custom GGs for some old Graflex cameras of mine, as well as a new GG for my 8x10 Canham, and 4x5 Arca-Swiss. They are fabulous. His GG is better than the GG that came with my Canham.

He's quick with his turn around, with good prices. I'm sure he can make an 8x20 for you, but better have him say that.

Good luck.