A 13/16" 6 pt deep spark plug socket has worked well for me. It's actually slightly smaller than 21mm (.8125 vs. 0.8268). I've got one that has both measurements inscribed on it.
IIRC I've heard most lug nuts are purposly sized in SAE equivalents: 17, 19, 21, & 23 MM or 11/16, 3/4, 13/16, 7/8.
Indeed! A 13/16" is a tighter fit compared to 21mm for Toyota applications and a 3/4" is a better fit compared to 19mm for my Hondas. I would use a 13/16" to further minimize lug nut swelling issue.
