There may be some confusion as to what people are calling a 405. The holder for the polaroid mp-4 can be fit to some cameras--others (most notably the graflex cameras), the holder will not fit without the lip and lock being filed off--very impractical and difficult to keep held shut even with tape. The graflex SLR camera will not fit any polaroid back unless it is used "upside down"--where you'd pull the film out from underneath--and, yes, the darkslide is difficult to pull from the graflex slrs. Agan--cutting off the lip and closure locking device will work, but then you need the graflock back pressure to hold the holder closed.
Ideally a design where there is no "lip"--or a lock that is "in reverse" will help tremendously. A big request a holder where the film is centered on the 4x5 frame like the polaroid mp-4 back. The normal 405 type backs have the frame against one side -- it is centered on the 4" dimension but not centered on the 5" dimension.
A holder for a 3x4 graflex (which is possible) would be the best new design to come around in years.