I made a basic drawstring bag for my Xootr last week. It uses a re-purposed shoulder strap from an old briefcase to carry the bike, with webbing continuing across the bottom of the bag for support. It's small when not in use, lightweight, and black....so grease won't show. Besides, it's easily washable. I made bags for the stem, seatpost, and handlebars out of a pair of old fleecy sweatpants and some adhesive velcro.
WalMart was having a Fabric sale at $1 a yard for Black polyester, that's what I used....2 yards.