I use black fiberglass window screen. Comes in a huge roll for about $5. If one layer isn't enough (I'm not sure the kind I use cuts PAR by that much with my LEDs but it does with T5s), you can use multiple layers of screen.
I like to use glass tops with my tanks and I just lay the screen across the glass top. If you can't do that, you could use tape to secure the screen. That way it'll be easy to remove. Since the fixture is gray, you could probably even use duct tape without it being noticeable.