The 305mm G Claron covers 7x17 with a bit to spare. I haven't tried it on 8x20. It may just cover. It's reasonably small, light (Coapl No. 1 shutter) easy to find and inexpensive.
Originally Posted by photo8x10
The 305mm Computar will cover 8x20 with plenty to spare. Unfortunately, it's a lot harder to find and much more expensive then the 305mm G Claron (Quality Camera currently has one listed at $1995.00).
If you move up to 355mm/14" the options become more plentiful. The 355mm G Claron covers 8x20 easily as does the 360mm/355mm Convertible Symmar. The latter is big and heavy, but is available in shutter (Compound or Ilex No. 5 for early samples, Copal No. 3 later).
Really Big Cameras