Others, probably the
majority, have large mushroom-headed studs on the racks that
mate with tapered slots on the side panels.
One tip - the bottom brackets on the back of the rack that hold up the
rear door often block the side panels from lifting. These brackets have to
be removed (two hex head bolts each) before you can remove the side panels.
As Ethan and others said, there are several different styles and not all
styles have these brackets, but you might want to check before resorting to
2x4s...
Bob