Why? All Macs have Time Machine built in as part of the OS, that incrementally backs up the computer every hour anyway. All you have to specify is a remote (i.e. network) drive space to do it to. This can be a NAS, or a DAS attached to a router. Apple even make an all in one solution for it in the form of a Time Capsule (early ones were 500gb, later models range from 1TB to 3TB), but I can understand if the OP doesn't want to start changing his network point/router set up. And if you don't want it backing up hourly, like I don't, then you can use TimeMachineEditor to schedule backups as you wish.

I would point out though, MacBooks do generally run pretty hot and the vents are around the screen hinge. So I wouldn't advise falling asleep with it in bed. That being said, when I'm really taxing mine and the fans are going crazy expelling the hot air, I've never encountered any thermal problems. But it's not to say it can't happen.