^^ This advice is exactly what I would say. I record 2-6 channels on my Mixpre-6 I with phantom power, full screen brightness, etc.
I use 4X Eneloop Pro's (Ikea LADDA) for <60 minute recordings just because it's easy/nice form factor. I can get a couple hours max with the 4AA's but it makes me nervous over 1hour.
I use an Anker USB-C 10,000mah battery for anything longer. That lasts for *hours*, but if you accidentally pull the cable out you're screwed, so I always still have some AA batteries in the machine when doing this as a backup (even Alkaline is OK---they'll give you 20 minutes of backup power to notice a problem with your USB-C cable)/
I keep the Anker battery attached to the Mixpre with 2 rubber bands. Best system I have found by far, and I have a ton of clamps/adapters. You can also use Velcro, which I do sometimes, but I don't love velcro glue on my equipment.
PS Powering the MixPre 10 with USB is amazing---this is the only reason I don't run a MixPre-10 (I really like the USB-powering options on the 6)