I used the black plastic screen that's made for screen porches. I had some black styrofoam pots / planters, and cut them into rings, then used fishing line to sew the baskets and attach them to the rings.
In another scenerio, I used PVC pipe, and made a rectangle "corral" and sewed the screening onto that.
Two winters ago, I had TONS of hornwort, about 12"-18" deep across the entire surface of my pond. I guess that the fish eventually learned they could eat it, because I can't keep it in the pond anymore.