Ya can't beat an old tire but if you're going to build a wood one, put some gussets from the bases to support the sides so they don't fold over. Don't make it much higher than necessary - maybe 1/2" higher than the pan is deep. Flop those 2x4's over and it'll give you a wide base for casters in the future. Also. you want the thing to be as wide as the engine is across the heads - for stability.