So I am running a Raspberry Pi 3, with an official 7" touchscreen, in a nice case that I got. Now I want to power it with a battery/phone charger. I have tested with a couple of the small chargers as below, but that gave me 1hr 43 min, and another about 2hours

Can any of you lovely people recommend a good powerbank/phone charger that will give me maybe 4 hours life to power the screen and the Pi ????
Thanks in advance