While Red Scholefield is probably the leading authority on batteries in RC, this is one of the instances where I disagree with him (partially).

Many people solder up their own battery packs, and while I wouldn't recommend it to someone without a lot of soldering experience, it can be done. If the nylon seal were as susceptable to heat as Red says, then the thousands of flyers making their own packs would be losing planes left and right. That isn't happening.

donoman,
I certainly would NOT attempt it with LiPoly cells though. They're extremely dangerous when abused, and I'd hate to see one blow up in your face.
Dennis-