I don't use this myself, but I know cycling instructors who strongly recommend this mirror
https://www.efficientvelo.com/home/safezone
It's large, mounts on helmet. For people with visual impairments, send email to the manufacturer. You can get extra lengths of the flexy mount arm thingy.
Be sure to mention why you are requesting extra lengths - the manufacturer does provide advice.
Someone in a forum who was blind in left eye used extra lengths to mount this mirror so that he/she could use right eye to look behind left shoulder.