Buy the external battery pack and plug in the USB-C to Magsafe 1 cable.

If you buy a battery with charge through capability then you can also charge the battery and the macbook at the same time.

Note the external battery I bought in the Parts list. I think there is even a thinner one available now. I payed about 70Eur/70USD

This external battery pack displays the volts, amps and estimated time left on the battery.