that's a tough one on where to put the nesting boxes. If it were me I would try and make it so the boxes are being supported by the studs of the building, but actually hovering in the air lower than the concrete blocks. Does that make sense

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Does it get cold there in the winter? If it does I would maybe considering getting at least 4 more chickens so that they can stay warmer
Bedding- In the past I've used sand in the summer and just sifted through it with rake to gather droppings. I've also used pine shavings and straw. In my opinion pine shavings soak up more moisture and therefore have to be cleaned/new added to more frequently. With the size of that building, I would probably consider using straw. Both do their job so there's not really a wrong answer! Good luck!