Looks like a good design to me (its almost exactly what I built
). I used 3/16" acrylic for the baffles and aquarium safe silicone to secure them. The only problem that I have, and would try to find a solution for is that the last baffle at the return pump bulges under pressure when I let evaporation get too bad. Some eggcrate bracing maybe. The water level will drop in that chamber as the water volume in the system is reduced, a good indicator for topping off. The pump may run dry if it gets too far. For me this can happen in 2 days.
Put a vent hole in your return line just below the water level in the display tank. This is to prevent siphoning if the power shuts off. The 6" headroom should be ok for the flow back. I planned the size of the pump chamber to dry out before the display tank spills over if the overflow stops returning water.
I have a homemade overflow. Unless you are absolutely confident in your DIY
skills, I would buy a factory built unit.