I have some preggy guppies and moved them to a seperate tank but they looked really sad, so I made up a device to keep the mothers in the original tank(where they are happier) and to keep the fry away from the mothers I brought a floating breeder and put a breeder net attached to the bottom with an elastic band. I dont know if it'll work in practice but it should in princible. As for keeping your breeder net still its a bit tricky...a new nets not too much money and you may find one thatsits better, have you tried clothes pegging it to a corner?