Go to a fabric store and look for the batting, which is what is inside the rainman i'm assuming. Get the thickest polyester one you can. Wrap it around the zepp until it is an inch or so thick. Then find something to hold it in place and you will be good to go.
Also, if the rain is really coming down, put some kind of glove over the cuff of your jacket or the rain will run down your arm and soak you from the inside of your jacket. I usually just use some of those black rubber serial killer gloves they have at Home Depot.
Good luck!
