I have built slatted racks in all of my bottom boards, beneath the frames and above the screen, though my bottom boards are closed (they are not entrances). I've also built slatted racks mounted vertically on the inside walls of some of my supers, especially those I use for brood, the vertical racks are thick enough that the pair of them take the same space as one frame, this still gives me enough room to fit eight frames in my 8-frame hives. I've been feeding pollen substitute and a few quarts of 1.5:1 sugar syrup to most of my colonies, including nucs. I've been making some modifications to my supers and swapping out the supers without the mods for those I've already modified and when I lift the bottom supers off of their slatted and screened bottom boards, all have had many bees clustered on the slatted racks beneath the frames. I believe this indicates that my bees are certainly taking advantage of the clustering space the slatted racks afford them beneath the bottom-most combs.
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I have also experimented with different configurations, designs, and materials for my bottom slatted racks. Some are parallel PVC pipe, set into holes drilled inside each end of the bottom board, some are redwood slats of different widths and orientations (side to side, end to end). Some of the slats are wide, some are narrow, some slats are oriented taller than they are thick (those look similar to rows of dominoes laid end to end and set on their longest narrow edges).