I've made a DIY LiFePO4 car battery to replace my aging 12V Lead Acid battery. It uses some recycled 5000 Amphour LeFePO4 cells from old e-bikes.

This box fits in the battery tray of my 2011 Leaf. 

I printed my one in PLA because that's my preferred material but this application probably suits ABS much better (ABS is less brittle than PLA).

My project page with more details: http://www.myonlinediary.com/index.php/Energy/DIYLifepo12VoltCarBattery

Update: 17 Feb 2021

Newer lid has larger holes to accommmodate 5mm thick spacers. These are needed when using a standard car battery lug. Also the holes are placed closer to the edge so that the connections will fit the car power blocks correctly.

The old lid has holes that work fine if you just want the main power cables to exit the box.