There is one slide that fits both sizes. No drilling is required but mind that whatever you hang from the edge of the bowl (there is also a BBQ tools 3 hooks hanger) your lid will not have a tight fit anymore. So for smoking you should remove the slide but for grilling it is fine.
although the lid might leak a little i have never had any problems smoking on mine. but yea, taking it off won't hurt. yes, weber still sells it and its very easy to install and uninstall.
Ken,
Yes. Mine arrived about a week ago and it works great. It does result in a bit of leakage but nothing significant. The only thing I've noticed is that there seems be fewer leftover coals when I shut down after a cook to save coals. No drilling required. Mounts in about the time it takes you to lay it down.