Was trawling through Amazon lightning deals and found this.
Seems like a good price… bare in mind though it seems like its for the 1st gen google home minis, not the newer google nest mini 2019 on.
Details from listings:
Aesthetic Design - Aesthetically pleasing design with a great silicone soft touch coating, takes nothing away from the audio quality, also adds an extra protection for your Home Mini. A really cool and clean accessory for Home Mini. (Not Compatible with Google Nest mini 2019.Google Mini is not included)
Enjoy Home Mini Anywhere - Comes with a wrist strap, you can easily carry or hang Mini from room to room while cleaning, or in the bathroom while taking a bath to enjoy Home Mini experience.
Easy to Use - Easy to install and remove. The precise openings on the case allow for easy access to all buttons and port, four LED battery indicator lights indicate battery level at a glance.
Real Portable Home Mini - 7800mAh battery provides up to 12 hours of run time (depends on its working status), so you can enjoy it at home or just bring it outside without electricity outlet.(within your wifi range)
What You Get - 1Portable Battery Base + 1User Guide & Warranty. 12-month product warranty. *Note:Hold down the power button for 3 seconds to turn ON/OFF the battery, press the button for 1 second to turn ON/OFF the LED. If the battery is being charged the LED will be blinking all the time. Please use Google Mini power supply to charge the base.