Camp Chef makes a rib rack which is 15 x 13 x 4 which will hold the racks front to back on your shelves. I believe that each rack will hold 4 or 5 slabs from what I have seen when in use. 4" high would hold a St Louis cut slab in place.
The ones at Lowe's are similar just made from cheaper material.
Humphrey's DownEast Beast W/BBQ Guru