Line the inside of the plastic box with carpet foam and it will improve the sound!
Also, adding poly-fil (super cheap stuff used for stuffing pillows) is a GREAT way to improve the sound of any sealed or ported speaker enclosure. It also fools the speaker into thinking the enclosure is larger than it is. Improves frequency response and deadens the resonance of the enclosure, "tightening" up the entire frequency range. Fill the enclosure with poly-fil very loosely, you don't want it packed in there tight!
