I usually place it sideways under the hood, or in the back seat area with the inside portion facing downwards. Never had it move around enough to worry.
If you plan to pull some G's the car is a bit more solid with the top on, anyway. Not as fun though, so I usually leave my top at home. Actually, when I lived in sunny California, I did not even have wipers on the car - they just burnt up in the sun & I never drove in rain anyway...
