You could build 5 four foot by three foot baffles, frame them out in 1x5 and fill them with rigid fiberglass. Cover the faces of the frames in fabric (something durable and not reflective at high frequencies) and then put another frame of 1x5 around the outside to hide the glue or staples holding the fabric on.
With five panels you can build a dog house like Terry's, and you can just put the panels against the wall when not in use. Even slicker would be to make one side masonite or luan under the fabric and make them wall mountable. Then you can have a slightly resonant bass trap or a very absorbent panel which you can use to change your room's acoustics.
Just thinking...
tom