Several things to consider:

Either a C cell or D cell alkaline battery will give more voltage, therefore more "heat" than a nicad cell.

A D cell alkaline will last longer than a C cell alkaline, but won't put out more voltage.. it'll just put out the same voltage for a longer time.

Today, most ni-starters use a C cell Nimh battery or a C cell Nicad battery. Either way, they work fine unless the battery is low even though they put out slightly less voltage than an alkaline cell.

If a C or D cell alkaline battery, in your opinion, isn't enough to keep the gloplug lit, then you have other problems that you need to figure out first such as fuel mixture, or your glo plug may not be the best heat-range plug for your application.

If you run voltage more than 1.5v into your glo plug, all of a sudden, you'll be wondering why you keep burning out glo plugs.

You didn't describe the application you intend to use this onboard igniter for, but I can tell you that I and plenty of my friends have run 2-stroke AND 4-stroke engines in all positions from upright, to sideways, to upside down, whatever. While we have sometimes had to play around with trying different fuels or plugs in various heat ranges, we have always managed to come up with a combination that works great without the weight or complexity of an onboard igniter.

Highflight