I buy a cheap mosquito net and hang it at a height to allow enough length to weigh the edges down with rocks on the ground. I ensure the opening of the net is to the front and hold that together with ordinary clip-style clothes pegs. If I have a lot of plants I can pull the edges of the net out quite a way, to ensure all the plants are covered.
I have had Monarch butterflies lay eggs on the net, anticipating that the new caterpillar will find its way to the plant, however one or two of those we can cope with.
I have heard that Monarch Butterflies can detect a milkweed plant from up to 3km (2 miles) away. 🦋