1 hour delay relay

Hi, I have an inverter in my van that I use to charge my drill, torch, etc. My problem is that I tend to forget to unplug the inverter when I'm finished so as a result I often end up with a dead battery the next morning. What I'm looking for a circuit that I can build that will hold a relay closed for 1 hour after the ignition is switched off. The reason for the 1 hour delay is so the inventor will remain on and continue to charge if I'm in a call and don't loose the charge time that would be lost if I just wired a relay to the ignition. Any help would be greatly appreciated Richard