Hi Jason. Welcome to HTF!
It's not really a good idea to cut a hole and shove a speaker into the opening and expect it to sound good.
Speakers radiate out the sides & back. Putting a speaker into a "cubby" in a entertainment center tends to produce a sound similar to you talking through a toilet-paper tube - a hollow/echo sound.
The Bose cubes work best attached to wall-mounts. They are prized for their small, unobtrusive size. Trying to hide them in a hole - is kind of redundant.
But if you must do this, try going to
www.partsexpress.com. This site has a lot of materials for speaker-builders. One thing you might look at is acoustically-transparent cloth (for making your own speaker cover), and sound-absorbtion panels/stuffing for filling in around the speakers to help reduce the echo.
Good Luck.