Protec or Specialised Accessories both make tailored fit covers to fit specific models of caravans. They will, or at least they used to, send samples of the materials used on request.
Some people like covers, some don't, you will get varying responses on the benefits or otherwise of covers.
I have a Protec cover with the easy fit option, simple enough to put on and take off although would recommend it as a two person task. Only used it for the first time last winter and although no major problems with it I'm not entirely convinced with it. I think all the cover manufacturers recommend the caravan be clean and dry before the cover goes on which can be a bit of a bind if you use your caravan regularly during the winter. Not always easy or convenient to get the van washed and dried. Protec recommend the van to be clean and dried but not polished or you might end up with the "tiger stripes", ie areas where the fabric has rubbed and polished the van up to a higher shine than surrounding areas. That did happen to mine to a very slight degree, I hadn't used a polish as such but a car shampoo/ polish combined. Not a great problem to polish the marks out but did defeat the purpose a little when I removed the cover on a perfectly clean van but needed to wash and polish it anyway to get rid of marks.
The main reasons I bought the cover were, stored close to trees, close to a main road that is heavily gritted in winter and prevailing wind direction means spray from the road in wet weather can be seen hitting the van