ZYBER Neck-Mounted Battery Pack, Compatible with Meta Quest 3S/Quest 3, 10000mAh 20W Fast-Charging Battery Pack, Provides Over 3 Hours of Extended VR Gaming time, Reduces Head Strain - White

It'll definitely beat the Index -- not having a tether at all is very freeing. Another option (other than some kind of aftermarket head strap) is a neck-mounted unit, something like this: https://www.amazon.com/ZYBER-Neck-Mounted-Compatible-10000mAh-Fast-Charging/dp/B0FL7RGKJL/