Abrade the surface of the LEDs, ideally with a gritblaster using fine media.
Edit: Bonus odd idea, PP or PE mouldings, eg.
www.evolab-th.com/16763423/96-well-nonskirted-pcr-plates
(, Mon 23 Aug 2021, 13:23, Reply)
Stick the leads through some thick foil and blast many in minutes.
(, Tue 24 Aug 2021, 19:18, Reply)