I did something similar with a old SCA cube. It was originally drilled for one drain and one return.
As said above, I just run it as a herbie with full gravity on the primary drain, which is seen in the 1st picture. The drain without a strainer (for photo purposes) is submerged prolly a inch or so to keep noise down. Drain with strainer is the emergency and is designed to make some noise when in use.
Ghetto return is self explanatory, lol. One day I'll paint it or something. Prolly gonna replace it for another low iron 50g cube or a 60g