This is my diy -not a sticker- nfc extender similar to the repeaters discussed here but much simpler and probably less efficient. Unless you have some tiny capacitors and magnet wire handy I wouldn’t bother. The gain was significant for me but you might as well wait for the stickers instead of making it yourself with no guarantees.
The other is a chinese usb-c wireless charging adaptor thingy which I made permanent by stripping as much thickness and sticking it iside the phone’s casing.