Looked at the cookshack rib rack, but just can't envision how you stack the ribs on there. As for the extra shelves, I found some cooling racks at a few restaurant supply stores for about $3 a piece although they're only 10x18. Cal sent me a PM with a link for a three slot side rails for the SM020 at the charcoal store and while checking it out I found their rib racks.
14x18 and will hold 7 full slabs per rack. Only bad part is the description says (ribs not included)
Might pick one up to try out.