I use 120 with spacers on the supply side, and made a special take up spool for t'other side. I could have just made some spacers with a "drive key", which would have meant I could shoot more than one roll..... I used the autographic window and masked that with a red window that lines up with the 6x6 numbers on the film backing paper.
I made some film guide rails out of brass sheet to support the narrower 120 film, and a "pressure plate" out of some card.
From memory (since it's a couple of years since I last shot with it), it ends up shooting 55 x 105mm frames or thereabouts.