The Slatted Racks from Betterbee are the best currently available from a comercial outlet, however, they leave a bit to be desired. The only good thing, compared to other slatted racks, is that the slats run the same direction as the frames. The Betterbee rack forces the bees to festoon under the slats so they are unable to effectively use it as a working platform. The racks I designed can be built with dowling or 1X1 that are actually 3/4X3/4 inches.
Make a shim 1 1/2 inches deep. Run the 1 inch dowling or the 1X1 trim the same direction as the frames. 1X2's (3/4X1 1/2) will work also. I plan out the position of the slats by measuring in 1/2 inch from each side of the end of the shim and then use an end bar from a frame for marking. Using the end bar gives proper spacing so the slat is directly under the frame and the eyelet in the end bar also allows the nail (screw) placement by using a ice pick or nail punch.
Mine allow the bees to use the rack with feet planted which enables them to not only use the area for expansion room due to heat or over crowding but as a working platform for ventilation or evaporation.