I could be wrong, but at that price range you will be hard pressed to do anything except a Parts Express knockdown box and value sub. I would consider saving a few extra pennies and checking with local innovation shops in your area. These are springing up more frequently across the US. Assuming you are from the US. If not, a circular saw, reciprocating saw, clamps and glue can be had on the used market very cheap to build a good sub. Take your time and measure a lot.
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