A box can be as easy as getting yourself a sheet of mdf, sealant and box of screws. You don't really need any specialized tools to build one. Every sub comes with a sheet that lists the recommended volume for the enclosure based on type (sealed, ported, band pass, etc.). From there you can figure out how much material ya need. The simpler the box, the easier it is to make, the less it's gonna cost.

A simple square box could be done for under $50.