I would close the holes, whether by welding or some other method, but a separate roof would really help reduce the heat level in there. But you cannot beat one of these for security.
My grandpa had large a steel tank on the farm that he had converted into a small grainery. (He had put an old refrigerator door into the side of it - the old type that latched mechanically.) When the tank got rust holes on top, he patched it with oil-base paint. First he'd lay a thick layer of paint over the area, then lay on a piece of heavy cloth, paint over that till it was soaked, do another layer, etc. Sort of like fiberglassing, only with paint & cloth. That survived the Oklahoma sun quite well.
