How about putting a battery operated door/window alarm on your fuel tank hatch. Scaring them off is preferable to a lock in my opinion. A lock will not wake you and all your neighbors up to witness the theives (who may have already siphoned out some of your neighbors fuel). I like these and have bought some (sold individually in some DollarTree Stores) to alarm some openings. http://www.dollartree.com/catalog/search.cmd?form_state=searchForm&keyword=door%20alarm They are a cheap alarm and I don't have to worry about losing the generic 751 key!
