Several decades ago slatted racks were advocated, not just for the bottom board but also inside supers in place of one frame. They provide additional clustering space for the bee population as well as channels to improve ventilation, those bees don't just hang out in this clustering space, they fan too. It was said to reduce swarming and provide other benefits to hive health. I notice that with them the queen will frequently lay even in the outside of the outer comb surfaces, a plus, 20 potential brood surfaces. They build very little burr comb in these spaces. I have tried both slatted bottom boards and slatted side racks. Colonies so equipped beard quit a bit less.
The racks are made by cutting 3/4" thick leftover scrap wood to 5/16-3/8" thickness (bee space) slats. Using polyurethane and small, wire frame nails I fasten two vertical slats 4" back from each end and then glue and nail on 5 horizontal slats spacing them 5/16" apart and leaving that much space from the end of each slat. I leave a bee space above the first slat and extra space remains beneath the 5th slat. This way they occupy the space of one frame (less weight) and provide extra space and ventilation for the bees.