Commercial batteries used for cameras and portable electronics are not rated for high amp. They are usually rated for something like 0.5 - 1 A or so. I checked once D-size 9000mAh NiMh battery and I was surprised to find out that it was rated only for 1A. Not enough for glow plug. Cold glow plug (depends on type) may take up to 3.5A. Like AVI8TOR said, cheapbatterypacks has good high discharge batteries that work for glow. I bought ELITE3600 form my on-board glow (two cells parallel). Now I see they have ELITE4000SC - should be a better fit. I wouldn't bother with ELITE4500, not for that price.

BTW, I have one more hour on my CoroCub, still the same glow plug. Looks like keeping glow plug powered during the flight doesn't hurt it at all. Now I'm flying with APC 16.5x5W, the engine starts by flipping the prop most of the time in right direction (I finally figured it out) and gets more power every flight (now loops are much easier to do). I can repeat after AVI8TOR : It's my favorite "knock around plane" now.