I was not sure if I could build a reasonable speaker enclosure with this material, but it was a lot of fun to try but also somewhat frustrating to build them. Wayne gave me suggestions on the construction, which helped a great deal. I did not have enough cardboard to attempt the Towers, although I might consider them if I can come up with enough material. The (somewhat) finished products are usable and sound great, but are very, very ugly. I'm trying to decide if I want to paint them.

A few comments:
Corrugated cardboard is very hard to work with. As an architect, I
built my share of corrugated cardboard models while in college, but
working with 5/8" thick panels is extremely difficult. Straight edge
cuts are almost impossible. Cutting the panels on a table saw produces
some really nasty saw(paper)dust.

I braced the boxes with quite a bit of wood, per Wayne's comments.
Bracing was glued and screwed or at least glued to the cardboard. I
used a lot of Elmer's white glue.

I spent a lot of time considering the front panel. Since one cannot
screw into cardboard, I had to devise a way to fasten the speakers. At
first I considered through bolting, but then realized that I would
have to cut open the boxes to ever get the speakers out. Then I designed many elaborate combinations of cardboard and wood anchoring points. In the end, I grabbed a sheet of 3/4" plywood sub-flooring that I had in my garage and just made wood front panels.

So, I've got the Pi Two's, and they work very well with my Bottlehead
Foreplay / SEX combination. They are as ugly as sin, but since they
will be providing music for me as I refinish my basement, I think I'll
leave them until the basement remodel is complete.