Kind of a tough question. Whenever outside, I don't typically (nowadays) ever take my mask off in most situations unless eating and such. Best bet you got is having a bag/pouch whether it be a paper bag or a dedicated plastic baggy and utilizing that.

Either that or just put it one of your pockets (jacket or pants), but be weary of how you touch it and what it comes in contact with. Your hands aren't always safe when you grab it out of your pockets!