How to add a 30 minute 110v timer into an existing wall heater?

My bathrooms are heated by in-wall electric heaters that are controlled by a rheostat type knob on the devices themselves.  I find that they often accidentally get left on for long periods of time (a safety and energy usage concern).  As such, I would like to add a timer into the existing devices that would cause them to shut off after 30 minutes (the timer would be reset by turning the heater off).  I know that they make wall switches which can accomplish this task, but due to limited wall space I would prefer to not have to wire a new switch to the heaters.  My goal is to find a small circuit board that I can add between the existing control knob and heater.  It would need to be small and heat resistant so that it could be placed inside the heater itself.  Anyway, I have been trying to find such a device online but haven't had any luck finding one that runs on 110v and can be added directly into the existing circuitry.  I was hoping someone on this site could point me in the right direction.

The heaters are both tied to the same breaker and are the only things on that circuit so another option would be to place the timer near the electrical panel.  However, this seems less desirable to me as you wouldn't be able to turn on the 2nd heater if the 1st heater had been left on and already tripped the timer.  Please correct me if I'm wrong about this.

You call always pull apart the wall switch mechanical timers. But a basic mechanical timer is easy to find. This switch is good up to 140F. The unit itself shouldn't be getting that hot near the controls so it will be fine.

http://www.ebay.com/itm/1pc-30-Minutes-Mechanical-...

If the control knob on the unit turns the unit on and off and adjust the heat then you can install this in series with the existing switch. Allowing you to level the original knob at the desired heat setting and the unit will not turn on till you spin the timer. Check the link and see the measurements of the timer. Keep in mind they are in millimeters. You shouldn't have any problem finding a space in the heater to add this. If you do then pull apart a wall switch. The timer in there should be a bit smaller.