For a really lightweight, easy to use cover, try the Storm Jacket.
http://www.stormjacket.com/It's basically a waterproof sleeve with bungies on the ends. It doesn't fully enclose, so no fogging up.
It doesn't work out-of-the-box with a tripod on a lens-mounted tripod ring, but you can bust the seem and (if you're good with a sewing machine) sew on some Velcro to keep it closed, or open it up and slide your tripod ring through.
If you're tripod mounting on the camera body, there should be plenty of extra material (depending on size), to slip it over the top and sides of the camera body.
For best results, you need to be using a decently-sized lens hood, so that might be problematic with UWA lenses.
John