I know that you said smooth muscle, but let me start with skeletal. Skeletal muscle looks like a bundle of fibers wrapped together. Making a model of that would be fairly straight forward. You could bundle together a fist full of straws from McDonald's. Better yet, bundle together a bunch of bungee cords. They even stretch out and contract like muscles.
Smooth muscle is not as straightforward, because smooth muscle fibers interlace to form sheets or layers of muscle tissue rather than bundles. If you are okay with cutting up some clothes, then I have simple model that should work. Take a pair of spandex compression shorts or yoga pants. Any kind of stretchy cloth will work. Cut it into strips. Make the strips 1 inch wide by 12 inches long. You can go longer, but I wouldn't go shorter. You will need a minimum of three strips. I recommend using more. Lay the strips out next to each other on a table, and weave/braid them together. What you get is a woven sheet (layer) of stretchy material. That's similar to smooth muscle, because it is interlaced muscles that form smooth sheets.