Our falcons are a foot wide in the middle. The first rack I built with ordinary L brackets for support was a little less wide and not very sturdy. Also a friend recently stored his glider with me for a while, so I figured it would be good to make a strong one for 3 gliders. Decided to have the middle support on a 2×4 post and the end supports bolted onto the existing cabinets. Put almost 8″ wide plywood on top of the supports. Turned out a decent rack.
Here is the post with pictures of the first rack: https://hgflier.wordpress.com/2012/05/06/gliders-are-home/