Oftentimes we buy a secondhand amp without a cover or our original cover has seen better days, well here are directions to a site that has details of how to make your own (Very Easy)
Once you buy or make expensive gear, you really want to protect it as its being moved around. Many times, you can buy travel covers when you buy gear new, however, many people often skip the expense because they would rather spend more on things that make sounds. If you buy gear used, odds are good that protective covers are no longer available, hard to come by or very costly. If you built the cabinets yourself, you probably have no choice but to make your own covers.
So go here:
http://www.colomar.com/Shavano/make_covers1.html
Regards Fenderphil