You might consider pouring a small pad with Sakrete or other pre-mixed stuff. I wouldn't think it would take more than 8-10 bags to pour a 4-5 inch thick pad two or three feet square. Put some scrap iron in it, let it dry for a couple of days, and you have a stable platform. Were it I, I would probably put it where the barn's going to be and either integrate it or eventually bury it. I'd also think you want to put plywood under the rear at the get-go to make it easier to roll, and to catch the occasional falling nut or pin or spring or whatever little thing might otherwise get lost in the grass.