For most home growers, "bagging" tomato flowers to keep insects from visiting them is fine (won't work for many other plants that require cross-pollination). You probably don't want to choose flowers which are intermingled with flowers of another tomato variety, but 20 ft. of separation should not be necessary.

I have used the little nylon net circles used to make wedding favors. You can get them at WalMart or party shops. A nylon stocking may also work as long as it doesn't heat up your flowers too much. You might want to ask for people's experiences at the Tomato Forum. Maybe you could title your post "preventing cross-pollination" or something similar so people don't think you're goal is creating a new hybrid.