Originally Posted by asmac

Sticking a steel cable through those pipes and filling them with coarse cement would be fairly cheap and would make them pretty much unbreakable and uncuttable with a saw or torch. I suppose a carbide or diamond grinder would eventually get through but it wouldn't be easy.

Edit: Apparently I should have read the whole thread before commenting. It's a good idea and apparently an obvious one to everyone except bike rack makers and customers.