Genuine Fishpaper (Barley paper/insulation paper) cut 70mm tall to fit around 20700 & 21700 cells perfectly. If you're not using cell holders, you must insulate between voltages (Between P groups) to avoid shorts through the thin PVC factory wraps cells come with.
Our vulcanized fish paper is highly abrasion-resistant, heat-resistant, and adhesive-backed for ease of use.
If you're using cell holders, you'll only need fishpaper on the tops and sides where you run balance cables, so a small amount is suitable. Otherwise, we reccomend about 5cm of fish paper per cell in your pack, + 20% or so extra to cover the edges of the entire pack and balance wire routing insulation.