They are economical storage units. I would put a cupilo on top for ventilation though. Nice dry hay has a lot of moisture. Our boy had one delivered a couple weeks ago, supposed to be rodent proof. His is yellow, forty footer, eight feet wide. I'm urging him to get gallon of red paint and put a red "velvetta" logo on side facing the road.$2200 delivered for a 40 footer high top conex...try to get a small (1/3 the size) shed delivered, or built for that price !
Old hay barns were built with spaces between the wooden side boards. I would worry hay would mold OR if hay not dry enough start a fire.The hay needs to be completely dry before putting it in the container. I put a load of bermuda squares in an enclosed trailer and left the rear doors open and the top vent cracked. Went to check it a few days later, water was dripping from the trailer ceiling inside. It needs enough air flow to allow the moisture to get away.