anyone familiar with chocking vehicles (like the fire dept does after an accident) knows that 2x4s and 4x4s are excellent equipment for holding vehicles. For example, 4x4s (actually 3.5x3.5) crossed over each other (kinda like those old lincoln logs) can hold 6000 lbs per crossing. So, if you crossed four 4x4s over each other, you could hold effectively 24000 lbs. Just make sure that the whole structure is square.