de Golden Busch lenses will cover a 16x20 - if you can find them. They're no longer made, but sometimes you see them for sale, usually at a hefty price.
http://www.superlarge.com/deGoldenBusch/index.html
(look at the bottom of the page)


I once owned two of them, the 482mm and the 1012mm (or whatever mm it is) and they easily covered my 12x20. However, they're big,
bulky and hard to filter. They were made by Mells Griot.