J-Rock,
Build the imperial out off exterior grade plywood which is ugly but cheap (er). Put some exterior grade paint on it to hide the ugliness. Parts Express has an eminence woofer on closeout that is priced right and actually has good specs for the Imperial;
http://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=299-388You could always go the cardboard route, but for outside that sounds iffy.
I understand wanting to build one (I've build two) and also the budget issue, but the other thing to think about is how much time you will invest in building and whether you want to invest that time to build something that's good or crappy. I thought about using cheap plywood (they are for PAs after all), but stumbled on to a deal with maple 13-ply for $28 a sheet. I think it took 5 sheets for a full size imperial so even with that awesome deal it's still $150 US for the wood. Then there's the glue, screws, biscuits, grills, wiring, binding posts, crossover, etc.
I'm certainly not trying to discourage you. I think with some skillful scrounging you could make one on the cheap and be happy with the results.
Good Luck!
Jim P