If you are stitchy-handy, cut off the legs of old jeans (at least 18"), use other parts of the jeans to make a strap, fill with sand or fine gravel and sew shut. Get carabiners and some rope, so you can clip them to the frame just under the canopy. (The strap is to be like the strap of a purse, and you use the carabiner to attach the strap to the rope)
I saw someone at a market who did that, and she said it worked well. It also started a lot of conversations, which is usually a good thing.