Update - 15 January 2017
I added a 5X stacked version and separators for the 3,4 and 5x stack packs. The separators make assembly much easier. I also added alternate versions of the inline with different port configurations.
Update - 3 January 2017
I increased the size of the wire port to make assembly easier, shortened the sides and added a little more clearance for the wires. I also increased the slot size for larger wire ties.
Over the last few years I've built a few 18650 LiIon packs which work very well in multi-rotors and very long range fixed wing planes. In any case I have tried several different ideas for end caps and these are the latest versions of what I have come up with.
There are two caps for each configuration, A & B. The 'A' caps have 1.5mm of clearance above the cell edge to accommodate a welded or soldered tab. The 'B' cap has 3.5mm of clearance and an 8mm x 3mm end port to accommodate up to 12Ga silicone wires plus the balance wires.
Both A & B caps also have small slots to use wire ties to hold them together around the cells. On the stacked 3 and 4 cell configurations you can also drill a 2mm hole through the center to use a piece of all thread. You can also simply glue them with hot glue. Lastly, shrink tube can then be used to fully protect the pack once the assembly is completed.