The official Shimano tool for this is all of $5, or if you want to get it from Jeff Bezos in which case it’s less than $7. I think one even came with one of my cranks, though that would have been ~10 years ago.

Should it probably be a common thing like a 6 mm allen? Yes. Can it be improvised with common toolbox items? Yes. Do I use a torque wrench with that particular plastic screw? No.