Are your halides and power compacts in one fixture? I wasn't sure when I read your post. If they are, you can suspend the fixture from the ceiling using stainless steel wire. It will give it a clean, nice look. Depending on how low you set your light, you may not get that much 'flooding outwards'. There are even retractable kits for hanging pendants and fixtures. Check Marine Depot and HelloLights.

As for the fabric, I wouldn't put fabric right on the edge of the fixture. With the halides, those will slowly burn that fabric and create a fire hazard. If you still want to create a cover around it, try this. Get some sort of material that you can trace the outline of the tank with. I'm thinking egg-crate but not sure if you can buy it in a sheet that big. Once the outline is cut out, attach it to the top of your pendant. Then attach your fabric to the sides, leaving a good majority of the top still uncovered. This will still allow the heat to escape. If your pendant is smaller width and lengthwise to your tank, this should allow some room between the fabric and the pendant so it won't heat up.

Or if you don't want to attach it to the pendant, you can always suspend it separately from the ceiling.