I could be wrong or just inexperienced, but I've been playing Airsoft for five years and I think it would be a bad idea to make your own (as in exploding or fire bad idea). I think you would be better off buying that battery. Hope I helped
I've made my own battery pack for my M4. It was going to cost like $50 to buy a pre-made one that was only about 2500 mah, because mines the retarded nunchuck style. Nowadays, you can't find that battery style, since mines an '07 or '08 model (its a nunchuck with 2 bars of 3 cells, and a single cell dangling).
Only 1 battery shorted, and thats because part of the battery's shrinkwrap was torn when I bought it. 3800 mah 10C discharge rate cell being shorted = HOT! Left a nice 2nd degree burn on my thumb. So, if you decide to make your own pack, and want to buy Tenergy cells, buy at least 2 extra cells. 1 of mine came dented (manufacturing defect, since there was no damage on the wrapper), and the other was the one I was just talking about. But, when I sent an e-mail to the website I bought it from, they sent me replacement cells at no charge, after I showed them pictures. I might still have the dented cell, but I'm pretty sure I threw out the one that shorted, since it was obviously dangerous to use it.
~Brian