With the Zucchini/Courgette I usually blanch it in boiled water from the kettle for about 10 mins before putting it in the aquarium. I didn't used to then I read somewhere it assists with the fish digesting it, not sure if that is true, but I guess you could test and see which they eat more of?
I weight mine down with a piece of lead weight, but the stone and rubber band sounds a good move too. Interestingly with the led weight I find my Oto's attack it from top, under and at the side.