You can often find faulty drill packs and harvest the good cells from those and cobble together a pack. Charging it safely isn't easy but can be done. There are battery management systems you can integrate. It's difficult to put inside the seat tube but putting it in a custom container on the down tube works well and that's a great place to put the added weight.

Doesn’t sound like a good idea to me… a malfunctioning battery would turn that into a blowtorch aimed at your nether regions.