Quest 3 kiwi elite headstrap is really comfortable and cheap.
I had a powerbank with 20.000mah just laying around so i used it even tho it might be a bit overkill. 10.000mah would also be enough. In comparison the quest 3 has an 5000mah internal battery

I was surprised that the version 1 just worked flawlessly and with a perfect fit. It's made like a cage with two parts that you glue together, that you can just break and reprint if there is an issue.
With battery attached its slightly balanced to the back of the head (first picture with file), meaning the facial interface just floats infront you your nose and eyes, much better then having the dreaded VR marks around your face. I can wear it for many hours without any annoyance

Tips for assembly:
Glue all 4 sides at the same time, doing 2 and 2 does not work well.
You could also mount it by ditching the two straps on the sides and put one horizontally around the top strap, it already has an indentation(pic with file) that would fit if you cut the strap width-wise (pic with red markings) - Edit I am using this now since its better.
The velcro goes around the black part of the kiwi strap, under the padding. You will not feel the velcro.

Most of the time on amazon you can change the country code to your own re: from "de" to "co.uk" or "com" and it will show the same item

Kiwi elite headstrap
https://www.amazon.de/dp/B0CMW2WTNJ?ref=ppx_yo2ov_dt_b_fed_asin_title

25mm Velcro strap
https://www.amazon.de/dp/B07BYHKRWW?ref=ppx_yo2ov_dt_b_fed_asin_title&th=1

INIU powebank 20.000mah
https://www.amazon.de/dp/B07YPSZ566?ref=ppx_yo2ov_dt_b_fed_asin_title

Angle usb-c connector
https://www.amazon.de/dp/B0CL9SGLW4?ref=ppx_yo2ov_dt_b_fed_asin_title