Sump spaces?

[mark]

Nothing wrong with the skimmer in the front part.

Common layout is something like this:
-divide to in 1/4s lengthwise
-first 1/4, water in and skimmer compartment, followed by 3 baffles, 1 inch apart (over/under/over)
-middle 2 1/4s is refugium, followed by baffle (over)
-last 1/4 return pump compartment

The 1/4s are just to give an idea. The first compartment you size just big enough in width for your skimmer. The pump compartment bigger the better to allow for evaporation though can reduce in size if have ATO.