The main task is to simplify the construction of the antenna itself, using one of the plans below, for example:

The not-so-cheap route would be to get a kit from cantenna.com.

Even cheaper, however, would be to use the plans at http://www.oreillynet.com/cs/weblog/view/wlg/448

Then when the antenna itself is as compact and uncluttered as possible, attach it to something big, like a shoe box or a bucket. Seriously. If you find the right thing that fits on the seat of your car, or the dashboard, or in the back glass -- and is significantly larger in mass than the antenna itself, you should have no trouble controlling its position. I prefer a bucket with a few books in it myself. Attach to the object with something solid, like hanger wire or a clamp of some kind. Avoid duct tape for this.

Oh, and don't drive like a maniac while trying to pick up a hotspot.