MK: Smell for a stock car is easy. Get a piece of cow or sheep manure, and you're in business. I've never had anything to do with sheep, but fresh dried "cow pies" actually have very little odor, and what there is smells more like decomposing grass clippings than "solid waste".
We hauled many pickup loads of them from a feedlot to a couple acres of Pasco sand back in the early 70s. It was dry, from backing in the sun, and the liquid draining into the sand underneath; well powdered from the cows walking on it; and smelled more like dirt than anything. We drove right into the pens, with the cows, and shoveled the bed full. Had a terrible time keeping it from blowing out on the drive back!