No lower horizontal rail needed. The baskets are permanently mounted with clamps to the top rails only.
Ben's Journal: Review: Wald #582 Folding Rear Mounted Bike Basket (aka Mesh Wire Panniers)
But you need to make sure the vertical struts on the rack do not protrude further out than the horizontal rails. Otherwise you might not be able mount the clamps or you baskets might be hanging in an angle.