So how does the audio place do it right the first time???
Many "professional" box designers just wing it based on the known formulas. Better box designers can tweak based on years of trial and error in many different applications. Absolutely no substitute for experience.
you dont need 4 15's to do a 150. im doing a 49.9 on music sealed at the dash with my 2 12's and a clamped 3500rms.
Implying that a Grand Cherokee is anything like a Wrangler with removeable top.
44.10$ a sheet of 4x8 5/8's mdf...I **** at building boxes
You might consider getting some designs from a reputable builder and finding a local finish carpenter or cabinet shop that can assemble from those plans if you want a clean look. It will likely be cheaper than shipping. Box is important, but for the price of shipping a good large box to Canada you could probably just take a vacation/road trip and pick it up yourself.
Also considering the arse **** you're going to take in shipping I'd look into lightweight equipment. Fi BTL neo subs would be my top choice since they're strong performers, well built and relatively lightweight for what they'll do. They will also hold good resale value. Personally I'd suggest 3 12's if you don't know what you're doing since they have a good bit of cone area and will be a lot less likely to fail on 5K than a pair of 15's or a single sub.
I would also look into Brazilian amps for the same reason. Stetsom or Soundigital 4K, or 5K could be shipped to you for far less money than any of the big Korean 5K's people are suggesting.
Don't cheap out on batteries or a strong HO alternator!!!
150 is a bit more difficult than most people think and I expect it'll cost well over 6000$ MSRP to do it with JL audio equipment and take up a lot more space than you're trying to use.
Also classifieds section is your friend. There's plenty of members in Canada that already took the tax hit to get things into the country (or just drove to USA to buy) that can probably save you a ton on good quality used gear.