My field camera cannot be collapsed with an installed lens. That leaves the bellows open to all manner of dust and flying debris, at put up and take down. I cut down an old plastic Kodak Grey Card to the size of the lens boards. I insert this whenever the lens has to be removed. It doubles as, wait for it, a grey card, useful occasionally in difficult lighting when I suspect my spot meter readings my be lying to me, or in color work.
GREAT idea! I can't fold any of my lenses into my Toyo either. I'm cutting down a gray card tonight. Thanks!