This was a fun challenge. I had thought about trying to make them stay in the goal, so when I first went on the forums here I wasn't surprised when I say that listed as a challenge. But I never would have thought about trying to get back to the construction zone. You do realize that you are evil, don't you?
(Now I am going to have set up a Paypal account just to get this silly game!)
The one thing about this challenge is that some of the solutions aren't really fixed in the construction zone....it's more of a temporary thing. For instance, if you let the Big Ball solution run for a little while (less than one game of solitare) the catapult gets pushed over the side and then the big ball and the target follow.
I would guess that the game designers never considered the possibility of catapolts when they were making the game. Once you get one of those working, a good number of the levels are really not much of a challenge. I started looking at coming up with something else to "replace" the catapolt solutions and started with the Big Ball. Everything crashed over the left edge while I was working on my version. I replayed the catapolt solution and had it running next to a game of solitare and watched it crash. In any event, here is my mobile car solution to the Big Ball:http://FantasticContraption.com/?designId=207790