i think it refers to the butterfly hunt
I'm a immovable obstacle --> can't touch the see-throught chest until u complete it
But if you fill me with wonder --> every time u grab a blue butterfly a "ghostly" chest falls into the one already there, therfore "filling" it
With the winning speed --> gotta be fast for the best prize
You'll claim the prize over yonder --> the chest is always a little walk away from the final butterfly