Are you sure of this? I've never had a problem with my P7 lights, even on high. Not when mounted on bike, anyways.

I've had them get hot when I used them as a flashlight -- but on a bike there's a 15 mph breeze blowing over them, and I've never found mine to get more than say ten degrees warmer than the outside air, even when on high. Perhaps during the day with 100 degree temps and the sun heating it as well it would be a problem (and get up to 120 degrees F or so) -- but normally you don't need your light then, or if you do, it's on flash and the average power is low.

And wrapping it up aluminum foil sounds counter productive. Yes, aluminum is a good conductor of heat, but your foil won't be well connected to the light itself (heat transfer works much like electrical conduction -- good joints make a difference) and so overall I'd expect it to keep the light warmer rather than cooler. Still, with a 15 mph airflow, I think it'll still work fine either way.