Yes, it is possible to build a small subwoofer although I never did, but I thought about it. The articles below and software spread sheets get into the weeds real quick, but you asked so here is one answer:
Back in May 1996, Matthew S. Polk (Polk Audio, Inc.) wrote an article in Audio Magazine about his HVCD Sub-Woofer (High Velocity Compression Drive). For a small fee, Matthew also supplied the spread sheet for calculating band pass box and driver parameters along with power port dimensions. The files and calculations are for personal use only.
First read the original Audio Magazine article - More Bass In Less Space by Matthew Polkhttp://wghwoodworking.com/audio/Audio_1996-HVCD-Subwoofer.pdf
Second, if you are still interested read the information and instructions supplied with the HVCD Sub-Woofer spreadsheethttp://wghwoodworking.com/audio/PolkHVCD.pdf
Third, if you have Microsoft Excel or Quattro Pro download the spreadsheets I copied from the original 3.5" floppy disks and start calculatinghttp://wghwoodworking.com/audio/POLK-HVCD-SUB-WOOFER.zip
You will be able to extend the bass even lower with a Marchand Bassis, a parametric equalizer that will extend the bass response of a small box and is a DIY alternative to the Behringer.
The April 1990 Audio magazine has a detailed article about the construction, adjustment and use of the Bassis by Ralph E. Gonzalez.
Here is a copy of the article in a pdf file for anyone wanting to build one of these units:http://wghwoodworking.com/audio/bassis.pdf
Bare boards used to be available, assembled Bassis are still listed on their websitehttp://www.marchandelec.com/wm8.html