Lrd Ka,
Welcome to the bb. Four months ago I made a refgium sump (see post below). I used a 20g tank and 1/4inc plexiglass partitions.
For ease of construction, and piece of mind, I would suggest you start from a tank rather than build the box. If you do build your own box be sure to get at least 1/4inc plexiglass.
I used aquarium sealant bought at my LFS (local fish store) to keep the partitions in. Be sure to follow the curing instructions.
Good luck.