Here's 3 Clean champions (red piece in the goal-all my built parts gone)
Big Ball: (based on 219435)http://FantasticContraption.com/?designId=221199
#51 (hidden level):http://FantasticContraption.com/?designId=215789
Obviously, stuff like Higher and Mars can't have the red ball stay alone in the goal.
A lot of the flat goals won't stop a ball, no matter how slowly it goes. The Wall looks do-able because of the square ball.
For instance, here's a (so very nearly) Clean Boomerang Champion for Mind the Gap:
(Solve the level, return the red piece to the construction area, leave it, it stays, and all the other construction falls off.http://FantasticContraption.com/?designId=235190
The problem is, the ball rolls off (in about 30 minutes!). So maybe you could put a time limit on it.