I've found fabric to be pretty expensive lately so if you're doing it just to save a few dollars you might be surprised.
I'd try one of the pre-made covers first, you can always return it if you don't like it. Bed Bath & Beyond has some and they often have 25% off coupons you can find in the junk mail. Jo-Ann's fabric also sends out coupons, so get on their mailing list if you're not already.