I don't use the cooking grate a whole lot in my open range, so it sits out next to the grill. I'm in AZ, so it's rare that we get rain....well we did and I got a bit of rust on the cooking grate now. I pulled out the bottom grate of the smoker box and put the firebox grate in there, and it does fit perfectly. I then just brushed it hard with a wire brush and rubbed some vegetable shortening on it, and it'll sit there and re-season during the next smoke and be good to go.
Oh, I should also mention that even though they fit in each other's places, don't put the smoker box grate into the firebox. It's not as heavy duty as the firebox grate, and it could possibly warp.