Since the 1st chamber of my sump had a baffle opening at the bottom I had an insert built out of acrylic. For a refugium it is important for the water to flow over the top. It also allowed me to make the refugium taller. I fashioned a channel to hold a removable egg crate filter. This keeps the chaeto from getting into the sump. The front is black to help contain overflow light.
This was the chamber holding the dreaded filter sock before the insert went in.
Here is the water flowing before adding chaeto. I didn't want any sand in my refugium, but wanted a good place for the copods and other critters to grow, so I added a few layers of marine pure cubes.
I bought a $45 led horticulture light off Amazon for the 12 x 14 " refugium and within a few weeks it was packed with 4-6 inch deep chaeto. The 150 watt LED light is super powerful (link below). I have to feed my 165 gallon reef tank pretty heavy to keep measurable amounts of Phosphate .02 ppm, and Nitrates 2 ppm. I will never change (or wash) a filter sock again.
UFO Led Grow Light, Growstar 150W
https://www.amazon.com/gp/product/B07312WKX2/ref=oh_aui_detailpage_o06_s00?ie=UTF8&psc=1