Ideas for sleds behind fourwheelers...?

We do this in winter and summer. We already have good sleds for winter. We need ideas for summer. Some are to thin so they hurt sliding across the ground. Some only go one direction (straight) so we can't drift good. They used something good in this video but I can't figure out what it is! http://www.youtube.com/watch?v=1e_N0ZW7G48