Great idea to demonstrate where to position the hand for thumbing.
Let students demonstrate in which hand the map feels more comfortable and natural and distribute thumbing cards based on that. In my experience not all right-handers hold their maps in their left hand. :-)
Granted, fewer people are left-handed and ambidextrous so you won't need as many of the right-hand-hold cards.
You are doing great work and I wish you success with your business model!