I only do this sort of thing in the darkroom with ortho film, but even with an enlarger it is surprisingly hard to get the right light level without a shutter. One of the reasons I sometimes mount one of my large format lenses in a Beseler sized lensboard.

If you want repeatable results, use a flash illuminated surface or the softbox. That also takes care of colour temperature. You could make a rough pinhole camera from a cardboard box with a reasonably long 'focal length' - say normal for the format. That gives you an aperture to work with and makes exposure calculations much simpler.

You can test it with strips of 35mm film - no need to waste sheets. Get an old cassette and wind the exposed film into that until you reach the main cassette, then draw out a fresh length. Just tell the lab not to cut the film or make prints 8-)

Though with a pinhole of known f-number and a flash meter you should be reasonably close first time.