So many variables can be at play here that I can't really answer the question about cheapest. What I can do is turn you on to something I have done with great success. I realized that custom hi-end car audio and pro/dj sound was the only type of custom cabinet that was available in almost every city in the US. This is huge for the non building DIY guy. You do have to find someone good though. When you see an insane car system, ignore the urge to hurl and focus on the trick custom work. If you see a great custom guitar cab or system in a club, compliment the sound guy and ask questions. In either case, when you see great work ask where it was created. Don't let the boomin' system sound put you off, the car install guys know how to make a strong box. They don't need to know beans about making it sound good(though they generally do, despite what it popular) That's your job to have a good design. You must have correct architectural drawings to scale. Have explicit notes on joinery, glueing, router work, recesses, ports etc. and make it clear that ANY amount of payment beyond the deposit (if they even want one)is contingent on following the plans accurately. Give them a couple of weeks, and tip them if it's nice. You will have a willing and cheap(especially compared to added shipping costs)source of boxes at your whim and just across town. Start with something cheap but difficult to asses the talent and if anybody gives you grief there are plenty more car guys around. In most shops people just drive around back and talk to the guys near quitting time or on lunch.(be quick at lunch) I have never met an installer that didn't take side work, especially in the off season after Christamas rush.To sum up: this isn't always cheap, but you to approve/revise before you pay for it. You can pay for nicer materials and construction instead of shipping and you can get pretty tricky with the designs, these guys have seen some weird shaped trunks, believe me. At a car shop there is an end to what the tools they have will do and the PA places are scarce but more well equipped. You don't want to pay for the shops that architectural designers use, but they can make anything you can draw, out of anything you can afford.
P.S. If you want special material (ten ply marine birch ply) bring it with you
Good luck,
Thomas