You could try a Schneider Symmar 360/620 convertible. Schneider advertised them as suitable for 30x40cm, i.e. 12"x16", so there is a good chance that they would cover 12x20 at closer distances. They come and go regularly on eBay and it shouldn't be too hard to get a nice one in a working Compound IV for under $500 if you are patient. Once the Compound shutters stopped production they went to a Copal 3 and the coverage declined. The earlier Dagor version of the 360 Symmar will have even greater coverage but they are a lot rarer so you could be waiting a long time for one of those.