Okay, make your car look weird or unexpected. The three times I've been broken into, I had just freshly cleaned out the car, and even vacuumed the carpets. As long as it looks messy and weird thieves seem to leave non-standard stuff alone. I don't know this to be a rule, but it seems to be a persistent coincidence. So, try something imaginative like build a fort out of cinder blocks around the back end of the van, and attach a wire leading back up to the business. Place a strong LED spot, cliplight illuminating the whole shebang. Stick a metal plate on the gas cap mad max style. Maybe they'll wonder if it's booby trapped. In short; create uncertainty.