Ive got a Panda cover on mine at the moment,
http://www.thepandacover.co.uk from ebay
the first Panda lasted approx 2 years in constant direct sunlight, this one has been on for 10 months and is as new but I did have the old one (no longer waterproof) over the top of it too save the exposure. I don't expect it too last past Summer 2017 so I'll need a new one for the 17/18 winter.
They are REASONABLE for £56 but are no way a long term money saver but they're the best Ive seen on eBay. First thing to go is the waterproofing on the inside so they start to leak strangely the material itself holds up and doesn't rot apart. Others Ive had from the USA not eBay have dissolved after 1 season and I managed to put my hand through them as if they are wet paper.
I've tried to get a made to measure all over cover but as part of my money vs life span theory at £56 every 2 years it is still 50% cheaper than spending £500 (quoted) for a cover that will only last 5-6 years. A word of warning though, Im told that a lot of cover makers 'on the net' are using the cheap Chinese fabrics but are still charging £400+ and they won't last over 3 years