good morning,
what I did for my RZ, as I couldn't find something ready made, was to freeze the camera on the tripod, stick a piece of white paper to the wall and make as many marks on the paper, until I had the exact framelines of the sensor. then I put some clear gel on the groundglas to copy those guide lines and copy them again to some black cardboard, that I covered with selfsticking gel. took some time, but helps a lot to frame right.
what I was looking for originally, was a mask, like I had it for my old RB for shooting 6x4,5, made of thin, black metal, that worked perfect, but nobody seems to provide these for different sensor sizes.
In my case the guidelines on the groundglass were not exactly in place, and I didn't want to be remembered all day of how much smaller my sensor is, compared to 6x7 ;-)
best regards,
marcel