Since the outhouse floor space is so limited and you want more than just a couple of banties, it seems to me the nestboxes should be on the exterior or you must cover them. Otherwise, they will be pooped in. Let's say they are in the interior.
I would not make the outhouse look like it has Dutch doors but still, cut the door so that the top or bottom can be opened separately. Then place 4 brackets on the interior walls to hold a 3/4 inch plywood floor about 30" (table height) above the ground. Making this floor 2' by 3' leaves room for a ladder so the birds can climb the 30".
From this floor, the birds will have no trouble jumping 12" onto a board. Placing another roosting board 12" inches off-set above that will make fairly good use of the area above the floor. I would use 2 by 4's and set them in notched brackets so that they could be easily pulled out for cleaning.
Directly below the floor could be 1 supersized nestbox, large enuf for 2 layers. Partitioning it may not be a good idea since it may make it difficult to reach the nests.
If the nestbox is nothing much more than a 12" shelf 18" above the ground, there will be room under it for the birds to move or to place a feeder/waterer.
My pop door is only 8" wide by 12" tall. That is snug but a little tight for large chickens so you may want their door a little larger.
just my 2 cents . . .
Steve