There is many power banks available for Leste devices, but I noticed that Hayden Power Bank 20 work with all Leste devices (tested N900, Droid4, Pinephone, Pinephone Keyboard), you can even charge it to half capacity with original Nokia N900 charger and then quick charge (PD) Pinephone Keyboard (it has enough capacity (full is 20000mAh) to both batterys 3000mAh + 6000 mAh) with it, remember put it to keyboard side of USB-connector not phone side connector.

It is also about same size than two Pinephones (Hayden 163x83x21mm and Pinephone 160,5x76.6x9.2mm)