That's indeed the first thing I thought of. The problem with the Pi0 was that it doesn't have a Display header, so it turned out to be very messy. The 7-inch touchscreen is very big anyway, so the Pi3 actually isn't that big and bulky. I'm sure if ideas like Crankshaft take off, there will be people who make the suction cup structure for you. For now you can buy the touch screen cover for the back, but it doesn't have the suction cup.

PS: If you really hate the bulkiness of the Pi3B, you can try the Pi A+. I haven't tried it because I don't have it, but it should also work.