sharke
Not exactly home-made, but if you're prepared to spend a hundred bucks or so then how about one of these?

It'll certainly kill some high frequencies and their reflections from three sides (there are three more, maybe two if you're close) but it can't do much for low frequencies.  A 100hz wave is 10 feet long.  It's a pressure variation that'll ooze around anything and through a lot of things.  And the wavelengths that fit nicely into your room will determine certain frequencies that'll be louder or softer and caused by the room itself.  There's nothing (easy) you can do about it.  Soft dialogue maybe.
 
You have to consider the ceiling because it's probably closer than anything else and you'll definitely get a reflection off of it, and not after 20ms which is the point at which it sounds ok (that's a wall 20 feet away).  So gobos like bitflipper describes (they're just moveable 'basstraps' which is handy) but you also want something similar on the ceiling above you.  Maybe a rug on the floor.
 
Use Room EQ Wizard to get a visual of what's going on at the point where the mic is.  A free easy to use program.  With it you can see the problem frequencies and reflections and set about dealing with them.  Low frequencies won't be easy.
 
I'm sitting in a room that's roughly 20' x 15' x 9' with a big opening into an adjacent room.  I haven't yet treated it for lack of time, but I'm looking forward to getting around to it.  The space is big enough that I think I'll be able to use certain reflections creatively (with diffusion).