Are downfiring subwoofer enclosures tuned differently that regular up firing woofers?

I'm making boxes for a JL 8'' sub and I was wondering if I could just make a lip around the side of the box that mounts the sub while still using the dimensions provided in the manual for the actual box. If the XMax for the sub is .39'', would a 1.5'' lip be adequate? Also, what is a good resource for DIY sub enclosure building for dummies?