Originally Posted by Velo Dog

For a one-shot deal, I'd just lay it down in the back. What's it going to hurt?
If you don't want to do that, though, I second the motion of a fork-mount holder bolted to a board. You can put it together for about 20 bucks, less if you already have a board, and it works great. I have four mounts on a 2x6, angled so the handlebars can overlap, and I've carried one to four bikes tens of thousands of miles that way.
But if I were only going to do it once, I'd still lay it down in the back.