The two long-term parking, P51 and P26 both come in at 101 CHF per week, so quite a lot less. There's no way they can be marked as full that far out. You'll have to check the day before. With the P26 you can reserve a space in advance for about 10-11 CHF all-in.
Both are about a 10 minute walk, or with P26 take a bus from the stop Arena for one stop to the airport.