Sounds like you want the 1/2 Imperial...4 of them.
If you look at the thread a few down entitled "Mr Valiant's Half Imperial" you will see some pictures of one that contains 4 x 8" speakers. You would want to build it to take one 15" speaker (but the idea of a removable baffle board so different speakers can be tried is a very good thing to have). The original (who's pictures are long gone) took one 15" speaker and looked almost like a full Imperial when you set two of them side by side. Supposedly they preformed as one big one too, with lower bass response, when two were places side by side. 1/2 width made them much easier to move around.
Here are some pics I found. The first. "open jam" has a full size Imperial on the left and two 1/2 Imperials on the right. The next pic shows the two 1/2 Imperials being moved after the show.