We had a nylon roof tent from MyWay, south African but much lighter than the traditional canvas ones.http://www.mywayrooftents.co.uk/evolution.html
http://www.mywayrooftents.co.uk/evolution.html
Used for 3 weeks through Morocco/Europe, great in the wind and on stony/sandy ground, or when scorpions are about. Much prefer a caravan in Britain - something to do with the weather :D
Those of us with Freelanders need to consider the weight high up - so nylon is the better option.
Don't know how it will fit on a roof rack with side rails as I just mounted it direct to the roof bars.

