I'd really like an external battery, also for my other devices.
My current favourite accessory is my
Book Seat. I use it for both my Kindle, tablet and Nintendo Switch (when I have detached the controllers). It's great for when reading in bed or on the sofa. I think it's too big for one a table or desk and have other holders for those situations.
It's fill with foam beads, so it easily adapts to having in on my lap and you can easily adjust the angle that way.