This is my 13s battery pack using 26650 batteries. I designed each cell to have some space, and airflow from the top and bottom. It will ideally draw air from the sides and act like a chimney pulling out the top. I also made it so the tabs could not be put on wrong as easily and the ends would not be prone to shorting out. Finally, I made tubes and paths for the BMS wires to each end along with various mounting holes and spots for 3mm thread inserts. It is a modular design, so if you want to make a bigger or smaller pack you can fairly easily. It stayed cool enough that even PLA did not melt. This creates a 48v(nominal) pack. I am using it to power my lawnmower. I will upload the bottom cover and top cap when I finish tweaking and testing them.
You can see many more details on my site: https://coolate.com/blog/blog/2018/07/21/a-new-battery-for-my-electric-lawn-mower-or-how-i-built-a-13s-26650-battery-pack