I got mine with the boat from Ron Lane at
http://www.fastlanesailing.com in San Diego. He has them made locally out of high-quality Sunbrella fabric. All of the I20's here have these covers for the jib, with almost all having similar ones for the tramp. They are very nice and I wouldn't get a cheaper one.
I don't know anything about the Murray's ones.
Alan Thompson
I20 - San Diego