Honestly sounds like a job for a Raspberry Pi Zero.
Small battery pack and a USB key, a bit of software setup, off you go.

Install Raspbian: https://www.raspberrypi.com/documentation/computers/getting-started.html#raspberry-pi-imager

Set it up as a hotspot:
https://www.raspberrypi.com/documentation/computers/configuration.html#enable-hotspot

Configure Samba share:
https://www.raspberrypi.com/documentation/computers/remote-access.html#samba