I double portage also.  On my first carry, I take my main pack, carry one paddle in one hand (for stability on corduroy) and a waterproof diver's box in the other hand (although it quite possible to carry both items in one hand - especially when there are bugs around!).

The diver's box is lightweight and completely waterproof (I've tested it), made of hard plastic, measures, 7" by 7" x 9" with no interior compartments and has a comfortable carrying strap. It has an easy snap-open pair of latches and contains my camera (a Canon point and shoot, not a DSLR), a high-powered pair of binoculars and my wallet.  It makes for very easy access to the camera and binoculars, both on the portage and in the canoe.

I don't know it's weight offhand but compared to the camera and binoculars it's weight contribution is minimal.