I've been hearing lots of good stuff about Sketch Up from Google. Good free software, probably fine for what you need. I wouldn't expect any other freeware cad apps to be any better.
You can check out CadVance as well, they are giving away their old 6.5 version that was current until the late 90s. It was formerly high end professional software back in the day so it's sure to be powerful enough for you.
CADVANCE 6.5 Form