Mac and Dennis have good points as well.

Another rather low cost source of absorbing panels (though clothes and wedges will do better) is to contact a local used office furniture/partition dealer. The patterns/materials on panels used to make 'cube farms' go out of style and end up at wholesale places. You can use some of those to make the walls non-parallel pretty easily. Here's one in Vancouver with their used acoustic panels listed:
http://www.buyritebc.com/used-products/modular-panel-systems/acoustic

The thing to keep in mind with these panels, however, is that they will not absorb mids and lows as well as something that has more depth to it, but they are certainly more durable than foam wedges, and if you don't have a bunch of overcoats and sweaters and blankets and sleeping bags to hang in that closet, they are a great solution. However, generally more expensive than buying purposed acoustic foam. First link www.foambymail.com is the absolute cheapest I have found over the years.

There's also great DIY instructions for building your own absorbing panels and bass traps at Ethan Winer's website.

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