Drill a small hole in the tailblock, before glassing, for a leash cord.
Through the entire tailblock, top to bottom? I wouldn’t want a cord sticking through the bottom of my board.
There are more elegant solutions. Glass leash loop
Drill an hole about 10mm,
with tape close the down side,
fill it with resin,
when resin is hard remove tape and sanding a little,
glassing and after drill it an 6 or 7 mm hole.
It could be a solution, or not?