My boxes shown above were built to fit the tubs. The tubs are 11 inches wide and 14 inches deep so yes 11 inches wide works.
Even my big fat wayandotte is broody in one as I type this.
I use dishpans in a wooden frame I built. Mine like them well enough that I have had constant broodies for over 3 months.
It cost me less than 60 to build and they really like it.