2 x 18's fit nicely if you add another baffle board (so the baffle board is now 1.5" thick). The new baffle board should go full width, so its 1.5" wider than the baffle board thats underneath. This buys you an extra 1.5" of wood for what remains after you cut the speaker holes out. And believe me, 1.5" makes all the difference in the world there. Here's the link to my picture...
http://www.mninter.net/~kuechle/imp08.jpgthat's dual 18's in the Imperial...notice how the front baffle board is full width and how little wood is left between and on the edges of the speakers. It is a standard size Imperial in every dimension, its just that a 2nd baffle board has been added. And when you do it, you will notice that the Imperial was almost designed to take a 2nd baffle board.
Also, be sure to try turning the Imperials around and firing them into a corner once you start figuring out where to place them. You will be amazed at how much more output you can get.
Dan