I think I might get 8x10 color film (if 5x7 isn't available) and cut it down to 5x7 size. The 2x6 shape is going to waste a lot of the 8x10 format anyway. At least 5x7 is more the right shape though still not perfect. 6x7cm would be good enough from a little viewing distance but probably wouldn't hold up for close viewing.
Use an 8x10 with divider boards, you'll waste less film and get two shots. Now that I think of it, that's possibly what I'd do. 1:3 is a hard aspect ratio.