About ten years ago, after losing a battery to theft, I bored a one-inch 
hole in hatch lid, using an Black & Decker and a nifty bit that cut 
circles.  I found a galvanized plumbing fitting and rubber washer that 
screwed together through/around hole.  I attached a 3/4" chain to the 
engine block, and now pass it through the hole and padlock on outside.  
It's not too elegant but it seems to ward off at least casual battery 
thieves.

Rob Murphy

PS:  Prime and paint around edge of bored hole to ward off rust demon.  
BTW, I did feel slightly violative of my bus as I bored the hole into its 
hatch, but it worked out fine.