Since I don't plan to remove my weights on a regular basis, I found a good-sized ammo can at the local Army/Navy store. I drilled four holes to match the bumper holes, ran 4 carriage bolts(with large washers) through, filled the can with about 100+ lbs. of used lead wheel weights and concrete. I let the concrete cure, painted it Rustoleum spruce green to match the Yanmar green chassis, and bolted it on. It isn't as nice as suitcase weights, but it's better than some other ideas I've tried..and much heavier than concrete alone!