Construction of inside of coop with roosts over droppings box:
front view of roosts over droppings box - there is a door thru the wall in the right side of droppings box for removal of dirty litter for composting

roosts lift up and droppings box opens from the top for filling with "clean" litter from the floor.
side view
with trim around bottom of droppings box and shavings added - the litter in the coop stays very clean - altho not quite this clean ;)