What setup do leafcutter ants need to keep their fungus garden healthy?
For some, one that's multi-chamber, so the ants can dump their waste "underground", away from the queen, brood, and fungus garden. Certain species of Leafcutter Ants do this in the wild and seem to have a difficult time adapting to dumping their trash in the foraging area in captivity, when forced. This applies to most Atta spp. and Acromyrmex spp., such as Atta texana.
For others, you can use a really simplistic setup that consists of the nest and the foraging area. This applies to most, if not all, of the more "primitive" Leafcutters, such as Trachymyrmex spp., Mycetomoellerius spp., etc.
All Leafcutter Ants need a nest that stays consistently humid. Your ants can tolerate dry conditions better than their fungus can, and their garden will quickly perish if humidity drops too low. The Conservatory, our nest built for Fungus-Farming Ants, has a humidity tray that keeps the garden humid for months at a time.