i'm pretty sure you can't flip faces or create flat planes in hammer. basically all that is left then is a complex cube maze. this is killing me on the inside because i really want to start making interior maps but getting the walls to align to the interior and secrets (basically making it so there are not weird gaps in the house, see: bathroom) without making it look weird is really annoying.
using the null texture i managed to make it look similar, but it is still two sided unlike the bedroom walls.