Green plastic mesh - think 'scaffolding protection' or wind screening for gardens. Relatively lightweight, rolls up easily and allows any rain to drain away. Same material is also useful for providing shade - as seen on continental sites. Materials sold to 'allow the ground to breathe' tend to be rubberised and quite heavy ... and expensive.
If you happen to be travelling in areas where olives are grown and harvested - e.g. Greece, Italy, Spain - lightweight black plastic mesh is used to cover the ground to collect the falling fruit and so, is easily obtainable from local hardware stores at sensible prices.