Excellent point Ken. We've seen it numerous posts right here that heat will destroy batteries. Even the tutorials on pack building state it.
A few suggest such batteries (2500MaH) would not provide the current necessary for electric flight, or not a long flight.

All in all, it appears, if you choose to try:

The majority use an iron 50 watts or above.
The iron should not be on the batter longer than 3-5 seconds to make the connection.
Holding the bar/wire down on the post with a screwdriver helps drain off excess heat (like a heatsink.)
Flux can be used, and tinning the battery and the wire or attaching bar is often suggested.
Also cleaning the battery posts with alcohol, or light scuff the terminals with sandpaper.

I've never tried it. But I'm curious. (And you know what curious gets you?)
I would hope no one would rush to do it without some checking, and satisfying their own concerns.

I have to agree with you Ken. 100% reliability is what we are seeking. And the best chance we have is professionally built packs.
(Still there is that 1% percent stupidity factor in me that says, "Watch this!"

LOL