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Really anything will do. It's only in the closed stations that they are 'mandatory' and any kind of Buff or similar will be accepted in reality. If you are concerned about it being effective, then just take a pocket full of disposable masks and put it on only when necessary. When skiing, any face covering gets so damp that I think it would be wildly inneffective at protecting against virus transmission.
Does anyone have any recommendations for facecoverings/buffs that are certified for use in Swiss resorts?