I discovered that a couple of large Domke wraps work pretty well -- I hook the two together to create a complete wrap around the whole thing and if I do it right there is sometimes an end with a piece of velcro left to hook under the rail and up around the other side to hold in place. I keep toying with the idea of making a semi-fitted "sack" with a drawstring but I never seem to get that far.

I'll be curious to see what others do. At one point I was using my darkcloth, but I have since given that up for a reflex viewer.

BTW, I don't use a photo backpack -- just a regular top-loading backpack from REI. With a photo backpack I think I'd be less worried about it.