Have you ever tried one of the old Linhof Universal holders? When you pull the darkslide out, the bottom flap flips up - spring loaded. To unload the film you pull a lever which pushes it out so you get an easy grip.
I have a few of these in "metric" sizes - 9x12 and 13x18. I know they were made in 4x5" as well, but they seem to be rare. I'll trade you a few 9x12's if you'd like, so you can try them out.