I made a 2x3 adapter to fit inside my 4x5 carrier by developing two sheets of 4x5 film with the lights on (in a tray) fixing and washing, then cutting the correct size hole with an X-acto knife, then gluing one edge of the sandwich together. I'm sure you could do something similar with mat board or some type of plastic. It may be even easier with 35mm because you won't have to be so exact with the edges of the hole - the 35mm sprocket holes are wider than the borders on a sheet of 2x3 film.
juan