At Grill-Repair.com we build custom outdoor kitchens and fireplaces, fire pits, wall units and do various backyard design customizations. Often a customer will request a cover for something other than the grill, as most grill manufacturers do have a cover specifically made for their grill. I have used a place in new jersey called mels bbq covers for custom jobs. They are very durable and on the expensive side.
for more traditional use there are various options with standardized sizes. On our site we have economy covers that are made in the form of a bbq that are similar to a fitted tarp. We also have deluxe bbq covers that are heavy vinyl with a layer of padding on the inside so a falling roof tile or brick will have some resistance to lessen any possible damage.