i imagine you could use film in a holder,
but it wouldn't be the best route.
the focus screen slides inside the box,
so it would be hard, unless you pierced a hole into
the box and slid the film holder down from the top
i have a design like that , but it was a real PITA ..
the other problem is ... there is no shutter ( yet ! )
so the exposure times would be kind of too long ..
i too love paper negatives, almost more than film