Sorry to jump in so late here, but I have an alternative that I learned from Beth Kahkonen at Washington State University during their Field Day this past June.
Instead of making/buying a separate piece of equipment, try putting a western under your bottom brood box. The western needs to have either all new foundation in it, or foundation that has only been used as brood comb (so it needs to be dry). Do not use foundation that has drawn comb that was used for honey supers. Beth says if you do, the bees will have an increased need to feel they need to work the comb, defeating the purpose of providing extra space.
In our group its referred to as, "Beth's Basement" or "the rec room". After adding it to half my hives (I used new, undrawn foundation), I noticed a huge decrease in bearding (as in nonexistent on those hives) and decreased swarm cell building. I really like the concept because many of us have extra westerns around and this didn't involve acquiring yet another type of "exclusive use" equipment that I would have to find storage for.