For my 75 Gallon I bought a standard 20 Gallon tank and put a 1/4" plexiglas divider right in the middle.... I kind of wish I had moved it back a bit more now.... but it does ok.
The top of the plexi is 1-1/2 inches below the top of the tank (for water to flow over. I later drilled a couple holes an inch below that as well. At the top I carved out a few "grooves" with a Dremel.
From the out of my tank the line comes through 2 90 degree turns down to a T then 90 degrees down to sump side.
to the refugium side I connected a ball valve (to control how much water was flowing to the refugium) then a straight pipe with a cap on the end. I drilled about 12 -15 small holes in the pipe for water to gently "pee" out of into the refugium where it overflows into the sump side.
I attached pix... please excuse the chaos that is currently under my tank as I try to organize and work out skimmer issues.
The only things that bothered this setup was the sump side is a little too small for all my stuff, water from the tank outlet creates A LOT of bubbles and fizzing. I should have only reserved 1/3 of the 20 Gallon for a refugium. Top offs and changes are touchy. only a gallon or so of water REALLY changes the water level in the sump I mean by like 2 or 3 inches so I have lines drawn on the tank and when they hit a certain level (about every 2 days) I hook up my RO/DI to top off- an
auto top off setup would eliminate this issue.
To solve the TONS of bubbles I cut a hole in the bottom and top of a 2 Litre soda bottle and "surrounded" the outlet thus containing 80% of the bubbles and keeping them from my return pump and ultimately storming through my tank.
I am happy with this setup and would really only change a couple of little things. Soon the refugium will have Macro Algae and a little LR in it. Right now it's housing some fresh coral frags.
As a note I made the refugium line removable as well as a couple of other pieces so I can easily remove them to clean. I thought I may end up with silt or who knows what getting hung up in the capped pipe side.