Any of the process lenses 30" and more will cover 16X20. That includes Artars, Ronars, Apo-Tessars, Nikkors, etc . If you shop carefully you should be able to find one in barrel at a reasonable cost. I use a 30" Red Dot Artar with 20X24 and it covers nicely.
Protars and Dagors that cover 16X20 are highly sought and very pricey. A 24" Dagor will set you back almost as much as the 550 Schneider XXL, which is in fact a Dagor, though much better than the old ones. And I don't think you will find a 24" Dagor in a shutter, other than Packard. If you can find a Series V 590mm or 690mm get ready to mortgage the farm.
Sandy King