I recently came across a rather ingenious DIY project that I thought I would share with you all. Rob Gipman, a photographer from the Netherlands, built this neat DIY water resistant sleeve for his Canon 100-400L lens. The best part is that it is just a bit of fleece and a combination of a few different water resistant solutions.
Rob put this together for his Canon 100-400L, but since it is just fabric you could easily modify it a bit to fit your lens and camera combination. The water resistant solutions are cheap too, so for under $30 you should be able to build this yourself and have a very effective rain cover.
I am actually thinking of maybe doing this for my Nikon 300mm F4 and D7000 combination, though I would modify the cover to go over both the lens and the camera. The weather around here can get rather wet about every month except July and August, so a cheap weather cover like this could be a great addition to my arsenal.