If you can set up a "chimney" like air current, that can help. An opening at the top of the house allows hot air to leave, which can then draw cooler air from lower down, if there is an opening.
We have openings (covered with hardware cloth to keep predators out) all around the top of the duck house, and their sliding door can be left open on hot nights (there is a pen surrounding the door), so that cool air comes in the sliding door as it is pulled by hot air leaving near the roof.
Cool water, perhaps with a chunk of ice can help.
Insulation helps keep duck houses cool in the summer as well as warmer in winter.
Shading the house so that it doesn't get too hot to begin with can help.