You could always hang it on hooks that go in spaces between the hood and fender and the hatch and rear quarter, or make bumper extensions that slide off when they're not it use and have it on the passenger side of the car, NOT on top, but on the SIDE. You'd have to forget about passengers and make awefully WIDE right turns though.
Here are a couple of other ideas.
1. Use an inflatable one, like a tire inner tube or a kiddie pool, secured to the roof and slip the boat through it. Use straps or PVC pipes from the front and rear bumpers to the ends of the boat to keep it steady.
2. Home depot and other building supply stores carry a wide variety of commercial grade Velcro type stuff. You could always put velcro on the across the top of the boat, and on the top of the car, turn it upside down and just stick it to the top of the car.
3. If your boat has oar riggers (I don't know the proper terminology, but you know what I'm talking about.) Turn it upside down and tie a rope from one to the other going around the rear hatch at the hinges and use nylon straps fore and aft attached to the towing hooks in the front and back under the bumpers. (So that's what those **** hook loops are for...)
4. Check with your local U-Haul dealer and see if they still have any car top racks with suction cups on the feet that stick to the roof. Years ago, they used to carry them. They had straps that held them on by clampling to the rain gutters of the car.
5. Fabricate one from PVC or aluminum pipe or "handicapped shower chairs". The handicapped shower chairs have the suction cups mention above on the bottom of the legs.
Keep in mind, I'm not trying to make light of your dilema, just trying to figure out a feasible solution to it. I used to fly quarter scale radio controlled airplanes and had a similar problem with how to transport the "one pieced" wings. They weren't 27 feet long though.
Use the noodle, not the ones you use in a swimming pool (although those and some rope going through them will tie it to the roof) , and you'll come up with something you can problably market eventually. Once you make your million in sales, you can have someone else transport the boat for ya.
And remember, you'll have Zcar.com forums to thank for the ideas. (Royalties...Hint, Hint)
Warren