I have played around with three of the options mentioned: 1) cut dark slide, 2) splitter in a camera that easily accepts one, 3) hacked up a 4x10 replacement back for an Intrepid.
Of the first two easy approaches, I was happiest with the splitter, as that was easy to use and I found it a bit less annoying than swapping dark slides and I didn't have to permanently change anything. In usage, I like #3 the best, mostly for composition and not having to worry about making sure you're in the sweet spot of the lens, but it takes a bit more work (not all that difficult) and you have to find 4x10 holders from which to base your ground glass distance. If you find those (I picked up some used Mido holders from the FS forum here) then the rest is pretty easy -- it was a weekend's project for me, made from scrap wood, a piece of window glass, and a few small screws and finish nails.
Quick peek here:
IMG_7480, Spring back installed by Robert Brazile, on Flickr
Robert
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