Rhino3D. It's a NURBS modeller, so it would be ideal for building interiors. Like make a cube, cut out rooms, cut out doors, windows, then furnish. The demo of Rhino has 20 saves AFAIK, so you could grab it, learn how to make cubes and extruded polylines in the space of 2 minutes, then use that. I'm no modeller, but I could carve out a building interior in Rhino in no time at all - it's the only modelling package I've ever felt creative with, using shapes to cut out other shapes to cut bits out of a shape, who can't identify with that
. NURBS modellers work like CSG systems, except efficient because your not using polygons, your using solid shapes - you make the polygon version later after specifying the polygon complexity. I use Rhino3D for all my modelling, everything from buildings to weapons to characters.
Health, Ammo, and bacon and eggs!