I have to chime in due to all the misinformation that's been given.
I know you've all done backyard hamstalls for years, but I've been in the ham mockup business most of my life. I have a B.S. in Hamgineering and have eaten everything I've ever made. Who can say that in this business?
Now, for ham boxes you'll want a Hogilloscope. This is the only proper way to measure the low frequency waves moving through the pork. I know everyone has an opinion on glaze, ham brand, cooking method etc. Opinions are like bacon...everybody has some.
Use the Hogilloscope and measure the ham's low frequencies that you intend to reproduce under your seat. Carving the fat from the ham will raise the frequency, injecting a meat marinade high in salt will lower the frequency.
Keep in mind that the space is limited under the seat. You may have to attach hog dog ports into the ham in order to get exactly the right resonance for your application.
If you intend to have massive amplification you may need to put some spare ribs around the ham to create greater stifness.
Seems like the advice has been pretty ham-fisted by people who haven't earned their chops.
Pork.