I have tried to put the plates in modern film holders. They have fit, but have been very hard to get out. I am thinking of modifying(just cutting off the guides of a cut film holder) one of my many junky holders, and putting some sort of adhesive on the back to hold the plate in. Anybody tried this?
Different film holders are constructed differently. If you have a holder that has little metal lips that the film slides under, it would be possible to take a small flat putty-knife, or possibly screw driver, and pry up the lips somewhat to give more clearance under them, they would be too loose for sheet film, but should then have enough clearance for tintype plate. I wouldn't try this with the plastic-lip type holders.