If you want something a little smaller, you might also try the Anker PowerCore 10000 PD. I think they are discontinued, but still available. They also have a PowerCore Slim 10000 PD that has been tempting me for a while. I think anything with PD should work with the appropriate cable. Also, make sure to get a charger for the battery that is PD compliant, otherwise you may face some LONG recharging times.
[EDIT:] It appears that the PowerCore 10000 PD is still listed on the Anker site. They introduced the PowerCore 10000 PD Redux at some point; I thought that supplanted the 10000 PD, but apparently not. I should also add that I get the full green power indicator on the MixPre with the 10000 PD, but I have never run more than four channels (never tried it with 5/6 plus the four XLR inputs).