We have had huge success with slatted racks in our apiary and recommend them in addition to a screened bottom. In spring and summer it gives the colony a place to cluster and is purported to reduce swarming. If nothing else it will keep your neighbors from freaking out when bees cluster on the entrance during that first heat wave. And the slats give the cluster a boost above the cold in the winter. We (and others) believe they are enormously helpful with overwintering in the north.
Forest Hill Woodworking has improved on the design that runs (sensibly) parallel to the frames - Ike beveled the top edges of the slats to improve mite drop. If you aren't using a slatted bottom, you really should consider it.
Available in 8 and 10 frame sizes.