Hi Solid, and welcome!
By far the easiest home subwoofer you can build is an infinite baffle sub. This type of sub doesn't have a sealed box -- it uses the world as its baffle (hence the "infinite" bit) and the thing that separates the front from the back of the driver is your house.
No box dimensions to worry about, etc etc, and by far the best bass you will ever hear. Check out the forum
here, or head over to
Cult of the Infinitely Baffled.