Are you looking for a bag for the doll, or a bag to carry a doll in?
A hobby isn't supposed to leave you wistful, regretful and vaguely disappointed. BJD collecting is as customizable as the dolls themselves. Do whatever pleases you the most, because otherwise, why do it at all? All for the 'Gram Bitches love the 'Gram.
I recommend going on the internet and finding a free pattern that is human sized and then downsizing it to 20% - 25% (depends on how big you want it in proportion to your doll). Works pretty darn good