This might win the "dumbest question of the year" award, but is there any reason to design a cab differently if you know it's just going to be used at bedroom or guitar-bass basement jam levels? I'm planning to build a pine 1 x 12 for said use but haven't picked a design. I'm thinking open-back, but how big? The amp is a 15-watt Spitfire clone.
For bedroom, I'd do closed back so I know where the sound is going. Then get some sound proofing to absorb some volume.
You'd be surprised how loud it is behind a cab.