I wish there were a way to put the chip & antenna inside (or behind?) the watch casing.
While this is really impressive, putting something in front of the faceplate breaks the great F-91W look and feel for me.
Projects like ollee Watch seem to be able to stash it behind the front crystal or underneath the LCD.
Or just forget the watch and casing and build an add-on to the strap to hold the NFC chip. This should work with any watch strap that isn't metal.
Similar project I did to carry a microSD memory card.
https://www.thingiverse.com/thing:6784665