Yes I have used sleeping bags too which work pretty well, If you are buying a std type new doll the box they come in is OK as a temporary solution until you get a fully operational system in place. The better quality higher budget ones tend to have shaped / moulded foam inside which lasts well - the cardboard will probably get tired but its fine in the short term.
Depending on your chosen storage area, if you don’t mind doing some basic woodwork its pretty simple to make a plywood box that fits under a bed or in a cupboard, make sure you make the inside nice and cozy for her with foam etc.
If you are ordering a new doll from a reliable supplier most of them also offer discounted price doll storage boxes as an accessory which she is then shipped in. - This will also ensure you get the right size!
If you want to spend some dosh on it and go super professional then check out
Sex Dolls Storage & Hiding - Coeros