Edited December 2021: To save you some time, this specific design is looong outdated. Your heat trap will not work if you copy this video. You can still build one but:
1: Pawns cannot build diagonally through walls anymore. You must build a proper path for them to seal the entrance to the furnace safely.
2: Sand bags must be built on every OTHER tile to slow raiders down as opposed to a continuous line of sandbags like in the video.