I would TOTALLY do the box one. I'm just not convinced that *I* could make the tube one work. Although an external amp would certainly simplify things. You can orient a box in any direction, and set things on it.
I think you'll have plenty of material. I might double the driver baffle (i.e. BOTH of the pieces you mention).
I want to build a sub someday. I keep coming back to the Rythmik ones. Maybe because I want to try a sealed sub to compare to my SVS PB12-ISD (also OLD, now).
Sounds like fun!
I love that picture. Kids grow fast. People try to tell you, but you just can't know until it happens to you.
bibere usque ad hilaritatem