Where can I buy one of these?
 

I'm in the planning stage of my new nano tank and I just need to find a internal overflow wall or box like the one attached.

Reefsupplies.ca carries them. Or you could make your own. I'm just finishing my own nano, I made an aio style tank out of a fluval ebi.

It needs to be 14" tall.
If I had a router I would make my own.

Do you need 2 or 3 sided? I make overflow boxes, and I can ship, pm me with dimensions and I'll get back to you with a price