First, the 210 SSXL will indeed cover 12X20. Not only will it cover, but you will have an inch or two of movements. But it is a very large lens, and you must use it with the center filter.
If you are interested in my 270mm Computar or 305mm Computar please contact me by pm. You can even write in French if you like as I used to speak French fluently and can still read it well. I believe you are French, no?