To keep the rain off I use clear plastic from home depot, you can get 2 lifetime supplies for about $30, I'd suggest a medium thickness, like 1.5 mil. and attach it with Gorilla Tape also available at home depot. It's like duct tape, only black and (the important part) it won't leave any sticky residue on your stuff. Not Sumo Tape, it leaves a residue, gotta be Gorilla tape. Gaffer tape falls off when it gets wet, so use it only in a pinch.
Or just buy all pro bodies and lenses with seals built in...

Hope that helps,
~Bill