Would tractor weights on some sort of bracket at each wheel be enough to hold it down, thinking of mobility when the ground freezes?
I like the outriggers idea now that someone mentioned freezing ground. Make them hinge on the sides over the wheels so they can be locked up or down at an angle, or make them slide out at both ends and then a 90 degree to the ground. I'd probably also make them adjustable for uneven ground. Someone get me a welder!