You may want to ascertian why the closer posiiton is so fickle, and instead of drafting for a position, draft the best players available. The cache you will have simply by going with best available will allow oyu to trade your prospects for a ml closer when you need. Otherwise, this strategy is doomed to:
A. leave you with a bunch of middle relievers at the ML level
b. leave you with no trading chips when you had the opportunity to draft a good player, not a player with a good position.
However, with that being said, I can understand why you want future closers. I would point to guys like:
Yorman Bazardo in DET
Mark Melancon - NYY (surgery, who cares, Mariano is signed for 3 years)
Chris Tillman - Orioles
Justin Masterson - Red Sox
Aaron Poreda - White Sox
and in doing so, you can look at appropriate hitting prospects, and see what I mean when you go bobbing for closers in the MiLB draft and chances are you will miss completely, and through no fault of your own.