Just because the henhouse is high off the ground won’t matter to the chickens if the roosts are only a few inches off the ground. Chicken like to roost on high roosts.
Yes make the nest boxes to fit on the outside. I would add onto the walls until the lowest part of the slanted roof would be 4 feet tall and make the tallest wall 5 feet tall. Although a slanted roof maybe easier to build a leaked roof would funnel all the air upwards and you could put vents on...
Add 3 nest boxes on one side of the henhouse and make the back wall open for easy access to clean, put the henhouse chicken door on the front wall near the wall with the nest boxes and then add some roosts on the wall opposite the nest boxes. Make the roof either slanted to one side or peaked...
Wait is this for chickens or ducks? If it’s for chickens then cut it down to 4 feet from the ground so that you don’t have to bend down low to clean out their bedding and you can also hang their feeders and waterer underneath.
Like my old henhouse: