I can relate, having had 3 posts go in the last 3 years; One was a very-hard to replace Peugeot size and I was not a happy bunny about that!!!

Something a guy I used to know with a Dahon did was drill a small hole in the last 1/2 inch or so of the tube then when the tube was down and protruding through the bottom of the frame, he could just attach a wee little padlock (a combination luggage one I believe) that kept the post in the frame and would dissuade most casual thieves. A bit easier to carry than extra cables etc as you can keep it on your key-fob.