There are a lot of pitfalls making your own battery packs, and as Opjose said, they are so reasonably priced, why bother.

One of the major factors is heat necessary in making the series connections for the packs. And, you do NOT want to use those little plastic holders with springs in them.

The internal seals in battery packs can be easily broken by heat if the heat is not properly applied when "soldering" the packs together, and if to much heat is used in the process.

And, because they are a series connection, one open in the path between the battery packs and you have basically a dead pack. No matter what anyone says, "I've done it and there is no problem", trust that you just can't afford to have a receiver pack go bad on you in flight, where the vibration can and will cause a poor connection to open on you causing a crash. Consider where that plane may crash if and when it does.

CGr.