Fans and vent slots usually work wonders.
I have a 12" tall canopy I built for my 250W MH
fixture that fits over my 20 gal
tank. I purchased a variable speed Icecap 120MM fan that is mounted to blow cool air into the canopy directly onto the bulb. Then hot air can excape either thru vent slots cut in the top of the canopy or out the back in the open space.