https://shop.pimoroni.com/products/adafruit-qt-py-rp2040?variant=39341945487443
https://shop.pimoroni.com/products/lipo-battery-pack?variant=20429081991
Doesn’t need to be complicated. A couple of tacticle buttons and if you want to get really swish you can pop a cap on them.
12mm if you want the caps.
https://shop.pimoroni.com/products/tactile-switches?variant=372242669
https://shop.pimoroni.com/products/colourful-switch-caps?variant=372246985
A bit of protoboard for the tactiles (would cut to size).
https://shop.pimoroni.com/products/pico-proto?variant=32369530110035
£26.70