For a WSM 18.5" - by best I mean the one that I can get the most into but also has big enough gaps to actually fit good spare ribs without having to squeeze them in (i.e. the rounded edge one from Webber themselves was to tight)
Filth! haha. I've only tried the Weber one and the big green egg one which was pretty good! I'm not generally a fan of racks though, I don't think they're very well designed. I wonder if a flat stacked design would be better?
It may do were you thinking of getting a couple of the cheap Homebase grills and separating them with Stainless nuts and bolts, which would double or treble the cooking area? Otherwise a purely vertical system wouldn't hold many ribs due to lid height.
Rack turned up and really happy with it as it is adjustable (anything from one rack to 6 and can move the uprights depending on the thickness of the ribs (i.e. beef or pork/babyback).