I went for the VW ones, don't know if there are more secure ones on the market but these seem ok. Act as a deterrent to the casual thief if nothing else - if someone really wants the wheels they'll have them off regardless.
I bought these from Halfrauds for £37, good quality. http://www.mcgard.com/index.php/automotive-2/wheel-locks/bolt-style
Wasn't sure which size to get, because there are quite a few. So I took a socket with me and whipped a bolt off the wheel and took it into shop to match up, the closest match was bolts about 3 or 4mm shorter than ones already fitted to van, but they are fine.