Yes, I bought fitted sheets to provide a soft dust cover in the garage.  WAX for those outside in the elements.  Next up I'm to build a bigger garage for the 2 LWB Minis to hide in.

Whatever you get, make sure it breathes. Moisture coming up from below gets trapped in some and destroys the paint. Some are hard and scratch.
I have mine in the garage with a sheet over it for dust and cats. I do the same with my Corvair that is in a carport.

Is this for permanent outdoor storage or incidental use?  Will the car be parked where it is "safe" or is there a chance that trees might drop stuff on the car?  

If you plan to leave the car covered most of the time and want a light-weight, rain proof cover, consider Covercraft covers made of their Weathershield product.  Their Noah fabrics are heavier (bulkier) to resist minor impacts.  The Noah products will last perhaps 2-3 years of constant outdoor use in heavy sun and weather.  For the same conditions the Weathershield will last 4-5 years.  

I have NOT bought their covers for the Mini but I have for some of my other cars.  The fit of custom covers has been very good.  You can buy directly from Covercraft but I suggest you look at the dealer list on their website and check their dealer prices.  The dealers are often priced less than the factory.

Looking for a good quality outdoor car cover for a classic mini... 76 Austin with rear antenna. Any suggestions/pictures/info is greatly appreciated. Thanks!