For many years, I've used foam sheets from Hobby Lobby (most hobby craft stores have these) to keep the refugium light off the rest of my gear. People often think they need colored or black acrylic in their sumps to avoid algae growth in other compartments and within their skimmer / reactors. However, that's really not the case. While we can see light in the other zones, what happens initially is the light bounces off the acrylic baffle and goes back into the refugium zone.
And as the macro algae grows and fills the zone, even that is obscured.
What is best is to block the way the light shines sideways through the air, above the baffles and waterline. Back in 2011, I found this foam and it was dirt cheap. $0.99 each, and I only needed three pieces for my project. The velcro cost more than the foam sheets. It held nicely, and I was done.