I have golf tees inserted in front of the tubs they cannot slide over then. It is easy and cheap, although there are better options out there. I know a guy whom uses this type,

http://www.eastloc.com/drawer_lock1.htm (bottom of the page)

He has killed a hole in the tub lip and has the drawer pin lock on the rack side so it will engage into the hole made in the tub. It is elegant and clean solution.