A bunch of us did a similar thing years back. We had an upturned car bonnet towed behind a proper 4x4 on a long rope in a field ... we were sitting not standing.
The acceleration coming out of a turn was quite scary, you almost overtook the tow vehicle. You could hardly see a thing for the snow thrown in you face by the front of the bonnet and I don't think I've ever been quite so cold, but it certainly beat run of the mill sledging!
