I'm not sure if it's the best since I only reached several hundred above 2000, but I reinforce the shorter beams that go up at the joints before the tower starts wobbling so it looks like the underside of a bridge on bothsides with the main otwer in the middle. Then I start building triangles instead of rectangles as I approach the clouds. You don't get much of a bonus, though.
Oddly enough, I have tried a wide variety of strategies, including the wide bottom, the very wide bottom, the flexible bottom, the thin as possible bottom, and the super reinforced tower. The end results are almost always the same. As long as you are careful, and (very) patient, and try to help support the red beams when possible, there doesn't seem to be much difference.
