The jumps in that video look awesome really nice and smooth. For a rough section we have a load of uncut door saddles (6ft long) which we lay out across the track staggered with some carpet over it.
We had planned on a crossover with a clear top but never got around to building it. We had been thinking of using a thick clear perspex with a mesh covering to give some grip should you end up driving over it.
Here's a video of the big tabletop at DiR, it's chicken wire table top which works ok but you cant drive over it particularly well as it sags too much.
http://www.youtube.com/watch?v=ERXtJ3QF8TI