I know this might be a struggle for some trying to build their own battery units, so I thought I’d start a thread that focused on the actual battery case. I have been hunting for a decent quality waterproof case that is small enough to fit into the hatch of my board. Peli-cases and the like are too large and not really designed for this purpose.
Here is one product that does meet the requirements: http://www.appbattery.com/product/ej48v15a-electric-car-lithium-48v-15ah-battery-box-power-plastic-shell-battery-car-18650-battery-waterproof-box/
The other option is to DIY the case, but its not exactly easy.