Buy an advertising sign. Printed on uv proof fabric and costs alomost nothing.
Often 6 metres x 12 metres. Lay over cat with mast down. Draw lines a little furhter out than wanted for a seemed edge. Draw in cutouts for bow, corners of stern etc and the stays and whatever. Take that one piece to an industrial sewer and a bit of velcro and a few grommits will create a very cheap, fitted cover.