First, make sure that the material is at least 3/4" thick, preferably MDF but quality plywood also works. Many use thicker material on the front baffle, which has been weakened by cutting holes in it.
The key is that the larger the box, the more bracing and deadening it needs to keep it from resonating in noticeable frequencies. I usually advise students to make sure that there are no spans over 12" without a brace.
Window braces like this one are excellent and can be made by piecing scrap together.
Deadening- if it is a ported box, use up to 1" of open cell foam such as carpet padding, the denser the better. If you are building a sealed box, use fiberglass or dacron pillow stuffing.
This is picture of a project from a couple of years ago with excellent deadening.
