What can I use to keep the heat in?

I have half my basement finished and when doing it they didnt put a door in the space to close off the room before going up the stairs. Theres an opening where a door shoulda went but didnt n its alil mishaped to put a door now. I currently have a blanket tacked up in the space but im making a gazillion tack holes bc it keeps fallin down. Should i suck it up n have my husband fill the holes when cold seasons over or is their n easier way thats not alot of money