A topic I am interested in!
I have the Behringer XENYX 1002B, which is a ‘big’, ‘proper’ mixer (with faders, sends etc.) that can run on batteries. I haven’t actually used it outside myself, but its a good fully-featured mixer, though it wouldn’t be an especially light or portable option with 6 of those big D batteries in it.
The Bastl Dude seems to suit a lot of people, I’d like one some day. A cheaper DIY option that I tried is the Rakit Rakimix, which a tiny active (9v battery) mixer. It has quite a few features (4x mono or 2x stereo channels, mute switches, an aux channel) for the size and price. My one doesn’t actually work that well, but I’m pretty sure that’s due to the poor quality of my soldering as Rakit kits are very good in my experience.