When you rent the car in Munich, ask them if they have a car with a Swiss registration for you. They are basically not allowed to give you such car, unless you guarantee them that you give it back in Switzerland.
They are usually very much interested to have somebody returning the car for them, therefore they may not ask you to pay the one way fee (which I guess is 200 EUR).
Anyhow, you need to be lucky, but it is worth to try.
Otherwise, I do not see a chance to avoid the one way fee.
(except of returning the car in Konstanz and take a train from there to Zurich)

Be careful: sometime they give you a Swiss car AND charge the one way fee, which I would not accept at all.