Petra,
I used the stacked plastic chair cover from Bunnings for many years.
It seemed to last at least three years or so.
Held in place with a bungee cord.
You probably know about this trick already: To prevent condensation damage, I put a box of "Hydrosorbent" (brand) desiccant under the cover, and secure the bottom. I recharge it in the oven once a month, on a day that doesn't look rainy so I can have the cover opened for a few hours. Has worked nicely over Sydney's very wet La Nina year.
