tuning a ported box?

Once the box is built the only "easy" way to tune it is to adjust port lenghts (assuming round removable ports).  The port length will have an effect on the frequency that the box is centered around. 

However, you can also add rocks to the box to make it sound better (ie decrease displacement), you could add fiberfill to the box to make the box act as if it were a little bigger, or you could modify the port opening.

There may be other ways to tune a box, but those are the most obvious.  Again, it is important to tune the box with a box design software program BEFORE it is built so you don't have to make any major changes.