MS
I had a 17 foot Folbot that weiged about 85 lb. It was enormous. I transported it on a turbocharged Corvair coupe. My roofrack bars were only about 30" apart. I could touch 90 MPH with no problem from the boat. As long as the boat points the same direction as the car and is securly fastrened amidhip and has bow and stern lines attached not much can go wrong.
My present boat is 16 feet long and is transported on a Chevy HHR. I transported the boat on a 500 mile jaunt down I-71 and back last summer mostly at near 70 MPH with no trouble.
Next if you build a kayak to fit in your closet or for any other non-nautical reason you will be disappointed on the water. Kayaks are the only boat that has a thousand years of development on them. The Inuits built the boats as they did because it was necessary not because they were playing into a fad. If you arbitrarily decide to go a different route you are on your own.
A lot of the plastic boats are short because it's much cheaper to rotomold short boats and most of the market is made up of new paddlers who aren't to particular. Those that stick with paddling will eventually get a properly sized kayak.

: Weight is not the first concern. With a small car, I cannot carry anything
: longer than 10 to 12 feet. Water sources are not any where near, so
: driving 50 to 100 KMs with a long boat on top, at slow speeds would be a
: killing!

: This is the only reason I want to build a small Kayak.
: I sure do understand that moving was a problem with a 9' SOT Kayak that I
: tried in some place near. It certainly made me feel like beating some one
: when on shore! It was slow, and the heeling was massive, when winds are
: strong, heeling was upsetting us.

: Nick, Tom and others what do you think?
: What is the best option for me from here?
: How long, how much to compromise?