There are some really good books out there that cover speaker
enclosures, the library is bound (no pun intended) to have one.

The book I eventually purchased is called "Speakers for your Home and
Automobile" by Gordon McComb, Alvis J. Evans, and Eric J. Evans.

Don't forget air ports. Subwoofers need a lot of air, and they can be
damaged by the pressure difference between inside and outside at higher
volumes. (Don't ask me how I know)

Also, rather than fiberfill inside, I have found that carpet scraps work
wonderfully and are almost free. And probably easier to use than the cat.