I use a cheap $20 or less cover from Walmart. Had it 2 years now and no issues. Mine is stored inside though and the cover is primarily to keep dust off it. I've used it outside in the rain now and again, if it's a nice day but chance of rain I bring it with me to work. For outside storage I'd be inclined to build a shed or get one of those smaller heavy canvas tents. A cover used all the time outside will keep moisture in, you'd have to take it off and hang it to dry after every rain I'd imagine.