As I understand it the main danger in this case is horizontal winds and the debris they carry that causes the most problems.

While nice to have you don't really need overhead cover. Caught in a field you would seek shelter from these horizontal winds as best you can. A culvert below ground would be ideal. A ditch that allows you to get your body entirely below ground level would be pretty good and a shallow depression, assuming that is the best you can manage, far better than nothing. I have read that just laying flat on the ground with your head shielded with your arms can make a big difference in your chances.

I would use any of these but also keep an eye on any rainfall and flooding. It wouldn't do to survive a tornado by sheltering in a culvert only to be drown when a wall of water from rainfall a mile away comes through.

As demonstrated on TV recently, avoid underpasses or other terrain or features that might channel and magnify the wind speed.