Don’t rely on curtains to secure a load – they are only there to provide protection from the weather. Even if the load doesn’t fall off in transit, it could fall when the curtain is opened.
Goods transported on curtain-sided vehicles should be secured as if they were being transported on a flatbed.
This means that the load should be secured to the vehicle chassis, and not to the weather protection structure.
There is a European Standard for a curtain that forms part of the load restraint system, however this also requires a reinforced superstructure, which must be tested to the Standard, and clearly labelled.