I use Li-Ion battery packs in the bulk of my aircraft. Their increased capacity is a worthwhile trade off for their lower discharge rate, for my use cases. In this video I'll share the design and ideas behind my DIY battery system.
18650 Pack Separator can be found at: https://www.thingiverse.com/thing:6684095
Use at your own risk. Li-Ion batteries (and Li-Po batteries) contain remarkable amounts of power that can cause harm if misused, stored in an unsafe manner, charged incorrectly, or handled carelessly. Do not attempt to make a battery pack unless you are fully comfortable with accepting those risks, and have a firm understanding of what your doing. Please be careful.