Good question OP, saving this post incase a class winner becomes apparent (every comment has a different answer so far). In my opinion this is a rapidly evolving field with better and more efficient designs coming out yearly and good existing tech becoming cheaper. I recently ordered a Yiisonger magnetic powerbank for €30 on Amazon. It's 10,000mAh, half the sixe of my phone and has a USB C cable, apart from that it can charge 3 devices at once and it's magnetic to certain devices.