Normally, you cannot apply more than 1.2v using a Nicad 1200mah cell or 1.5v using a large dry cell. There are some 2.0v glow plugs intended for use with a single lead acid battery cell. Now to use 12v or 7.2v to heat up the glow plug means you need a electronic driver circuit that reduces the voltage and limits the current going to the glow plug. You commonly see these devices on the power panel for airplane flightboxes.
You can make one, or you can buy one. Sullivan has a onboard glow starter that is compact and should be easy enough to install in most any RC device, be it a car, boar or airplane. it works from 2.0v up to 13.6 volts, so it would work well with a 7.2v battery pack.
What the onboard glow starter does is use PWm (pulse width modulation) to apply the voltage and current to the glow plug. It senses the current flow and it knows the input voltage, so it can automatically adjust itself to provide enough power to heat up the glow plug without burning it out.