The standard Polaroid back will work - https://www.ebay.com/itm/133844203860
They're quite common and should not cost much given that the film is no longer in production.
the rb67 doesn't cover the format, you will have quite some black margins. seen the fact the 100 film is rare and has become precious, wouldn't it be a shame not to use a full format camera? I wish I were able to buy 10 packs at a reasonable price...