Not sure if you have room, but you are thinking in the box to much.(pun intended)
Cornish are not as lazy as you think. If you have room those 30 chickens need a wire fence about 20 ft wide by 60 long. feed them at the far end from the building water in the middle. They will go in every night and they don't cripple up near as much as when they are fed in a building.
The other way I find works well but more work. build a 10X20 fence with no in ground poles and move it every day or two. If you make it right you can pull it with a quad or garden tractor. on 1 end you need only as much roof as they need to get under when raining or night. Square tubing at the bottom and a few uprights and tubing on the top, 1 gate and you have a portable pen.
I had a 4X8 building on skids for 40 with just roosts in it and they all go in at night. They will roost just like any other chicken if you give them the roosts. You can close the door at night when they are all in and drag it before you open the door in the morning. When the get to 7-10 lbs it gets harder to pull around. They don't grow as fast but much healthier that way.
Having a level yard helps. I let mine run all over the yard now as I have a dog (X4) that keep all rodents away and don't touch the chickens.