answer:Does it have to be just large enough for a gym bag or can it be larger? Does the owner of the gym bag have significant financial means? If it can be a larger storage space and the owner is financially able, why not have him/her rent an apartment relatively far from his/her home? This could work if your character needs to change into the costume often, because then it wouldn’t be strange for someone to go in and out of an apartment a lot, and if he/she was absent for long periods of time, this could be explained away by your character masquerading as a business man who goes on lots of business trips. If the apartment is sparsely furnished and the landlord has to poke his head in, it could be explained away as a love nest. Furthermore, if you need some kind of conflict to arise around the gym bag, a landlord or maintenance man could stumble upon it when entering the apartment because of some emergency such as a burst pipe or structural damage. If your character does not have the financial capability to do this, perhaps he/she can hide the bag in an amusement park and find a secret way to sneak in. Once inside, he/she could blend in with the crowd until he/she got to the duffel bag.