Just for flop? I haven't tried this yet, but here's an idea that wouldn't have anything attached to the downtube, nothing to keep up with or buckle, not in the wind. I'd take a piece of tube rubber or other elastic and loop it tightly under the front brake mount (or attached to the brake bolt) and around the back of the fork under the crown race. Might need to rig a bracket to keep it from riding up to the head tube.

When stopped, just stretch it out and let it snap tight against the head tube. That should be enough to keep the wheel from flopping. I don't know how well this works - it should - but it would only take a minute to try it out.

edit: don't bother; I tried it and it didn't work very well.