Winter covers don't want to be oversize and flap about and damage the paintwork.
So it's best to get a dedicated Eriba cover, and when I bought mine in 2014 I approached Protec who make covers to fit , they said fine but there was a waiting list of 5 months ! So I got mine from AL approx £200.It has access by a zipped door, and straps underneath. It is now 4 years old, I have repaired it last year with spare fabric that came with it where it rubbed on the "lugs" for my awning on the rail. So I now bubble wrap any sharp projecting areas before I put it on, and remove the chimney, put a tennis ball in to keep the spiders out!
Or you could watch this forum they do come up for sale occasionally.

Some info here for you ....

Fitted winter covers

Hi deepc

We also got a winter cover from AL for our 540. It is made by Kampa and we have used it for two winters and are really pleased with it. We also take the boiler chimney off and seal it with bubble wrap so as not to damage the cover. Think ours cost just under £200. MikeT