Two options, both involving nylon stand offs that you can purchase at most any good hardware store or big box hardware store.
One solution is to take the bulb/tube/lamp out of the fixture and drill a couple of mounting holes through it. The ballast and wiring are in the back third of the fixture.
The other solution would be to gut the fixture and mount the reflector and retaining clips to the canopy. Both options would use the stand offs to allow air to flow between the canopy top and the reflector.