Tips to help keep a car warm when heaters don’t work?

I’ve been in this exact situation when I was around 18 and my first car’s heating stopped working. I ignored it because winter was just about over and then it was warm enough that I thought it didn’t matter and I could then fix it the next winter when I’d have more money. Turns out it did matter because a months later the head gasket blew while I was on the motorway and it cost me like £1000ish to get it fixed. Can’t remember the exact amount because it was around 15 years ago but it was more money than I had. I had to go begging to mum and dad to borrow the money to get it fixed and then basically every spare penny I earned for the next few months went to paying them back. Most painful way to learn to never ignore small issues on a car and get things fixed straight away before they become big issues. Hopefully this isn’t the problem OP has.