Good tip from Sean about the chequer plate. He's right about the folds though; if the plates are flexible and one of them starts to scrape the underside of the car you can hold it down, or at least know it won't do as much damage as a rigid plate. If you bend a plate with folds it is also much harder to straighten. Another place to look for suitable plates is in a lorry scrapyard; very often behind the cab of the big artic. tractor units there is some excellent perforated aluminium decking plate. Or for a bought solution, there are some roll up mats at http://www.equip-raid.fr/cat/P05/071.html
Happy trails
Luke