This design is for an old SVS driver in a 114 liter, 18" diameter sonotube, with a 4" flared port, tuned to around 20hz. The finished sub stands 36" tall

I used threaded rod since this was my first DIY sub.
I figured I'd like to be able to take it apart and "tweak" it if I wanted to.
Above is a look inside with the egg crate mattress pad that I used at first.  After finishing I couldn't help
but wonder if the foam was actually working like poly-fill or actually
taking up internal volume so for sanity sake I opted to replace it with poly-fill (below).
 

Old SVS driver installed.

Fully Assembled. I used Parts Express cherry vinyl, that I had left over from my flexi rack, to cover the top.
I think it looks much better than painted MDF.  I can thank my wife for the idea. (see picture below).
My wife still has to do her duty and make the sock for it.
 

The cherry vinyl looks much lighter in the picture than it really is.
You can see more of the actual color on top of the black sub next to the sonosub.
 

The base is two MDF circles glued together.  The legs about 2 inches long.
They are 1" PVC couplings painted black.  I bought them so I wouldn't have to worry about cutting
three sections of PVC to the exact height (and they were only 21 cents).

The sonosub next to its predecessor, the Polk Audio PSW450.