If you buy the glow driver from South Herts Models (Telco)
http://www.southhertsmodels.co.uk/ - it is electronic, fully automatic, small and light - The unit detects when the glow plug needs heating an supplies the correct amount of current. You need a separate battery but as the unit only supplies current when it is needed the battery does not have to be very big and lasts a long time between charges.
If you really need a glow driver - save yourself a lot of headaches and get one of these rather than messing around with some Heath Robinson affair!!
The sell for about $30-00 in the UK (they are available under a different name in the USA - Nelson) and have proved to be popular and reliable. Personally I am quite happy to spend a few extra $$$ for quality servos etc it it will keep an expensive plane flying.