I use a 355mm G-Claron and a 19" Goerz Red Dot Artar.
For the first few years of shooting with 12x20 I used the 355 G exclusively. The last few months I've been using the 19" RDA for a vast majority of shots (interiors still work better with the G claron) ... both cover the format surprisingly well and are great lenses, couple inches of coverage with the 355 and multiple inches with the 19" .
I found the 19" RDA for about $175 on ebay. Its barrel mounted but easy to manage with F90 and a lenscap.