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.

G'day Mate, you wont be disappointed if you build 'Half Imperials', if you follow my thread. The 18's may well require a slight rejig of the width i.e, the width of my 1/2 Imp would be an inch or so short of 18". Plus, my 1/2 Imperial with 4x8" drivers, passively crossed over with a VIFA D19 turned out to be a very 'pleasant' speaker for HiFi