According to his instructions, you would require 2x 12v CCTV batteries, a 3rd party MacBook car charger, soldering iron, flux, solder, heat shrink tubing and 5x blocking diode. These items are readily available at computer shops and hardware stores so getting access to them shouldn’t be a problem. The rest of the instructions are available at the source link below, along with a video where he goes on to further explain what else you need to do. Not just limited to MacBook computers, it seems that this DIY external battery should also be compatible with any device that has a car charger as one of its accessories.