a good base! I've been dreaming of welding up something like that, nice and sturdy, because I can't stand any of the ones at the store, that only have one or two little legs out back. Those always move around on the dirt until they fall over.
One of my friends tried making something like that, but he didn't weld the legs on very firmly, and one broke off the first time we took it out. Plus, he didn't lean the tree forward or put springs in it, so the arms would stop across once, then swing rearward and stop when pointing straight back. The only solution was to shoot the paddle immediately when it appeared on the other side to keep the momentum up!