This is a pain. It can't be too hard to design something that stays put, can it?
While I'm in the mood for a grumble, why can't Canon offer centre pinch lens caps, and provide a hood with all their lenses, or at least the more expensive ones? I suppose Canon see this as another source of income, but they're ridiculously expensive and it just seems to be profiteering. How many people actually buy the Canon hoods, and how many visit eBay for a cheaper solution?