The most popular heater is definitely going to be the Mr Buddy heater. I have camped in totally uninsulated tents in ten degrees in the snow with just a Mr Buddy and been very comfortable.
That said, the main limitation of the Mr Buddy heater is the very small one-pound propane tank that it comes with. If you want to leave it on full-blast all night, it’s going to take about four of these one-pound tanks. And you’re going to have to wake up and change the tanks when they run out. Luckily there are several ways to fix this problem.
Keep in mind this heater is rated as very safe for indoor use because it has auto-shutoff if it falls over, overheats, or detects carbon monoxide. It’s a super popular and very safe option for indoor heating.
First, you need to insulate the space. Hang up some cheap space blankets using safety pins or hang some reflectix. That’s going to drastically reduce the amount of heat you will need from your Mr Buddy heater.
Second, get a 20lb tank and use this simple adapter to connect it to your Mr Buddy! This is much cheaper because these tanks can be refilled unlike the 1lb tanks which only work once. Now you’re cooking with gas! This setup will last close to a week running full-blast. Much better than the 1lb tanks.