Bleh to rope caulk. It works great but not if you ever want to get the drivers back out without destroying them or the box. I bought the Dayton BR-S1 kit, which came with rope caulk. After putting them together I wanted to double check the crossovers to make sure there were no problems, but even after shoving a broom handle through the back and pressing on the back of the driver about as hard as I dared to, I could not get it out. Fortunately I think I got everything right, after they broke in they sounded pretty good. But had I not I would probably be screwed.
On my next build I just put a small bead of silicon, smoothed it with my finger and let it dry, it made a nice rubbery seal. Weather stripping tape works well also from what I hear.