We had a Protec cover for our previous Eriba, it fitted perfectly, kept the rain off, etc. But it didn't succeed in keeping the van clean. After the winter months, there was always a fine coating of sticky dust on the roof (probably algae) which was fun to clean off.
With our new slightly larger Eriba Triton we decided not to bother with a cover, first of all Eriba's don't leak and secondly it takes the same time to clean the van whether covered on not (for us).
Robert