where do you live ?
I can think of a number of ways to do this.
But, as with all things customised it's bloody difficult without the person's limbs in attendance :-)

Something as simple as a basic exo skeletal 'glove' with elastic bands, would probably work. And the beauty of elastic bands is that you can easily adjust tension and they are dirt cheap and simple to change or replace.

what 3d printer do you have ?

And have you got someone to do any fine fitting or toolwork when making the 'gloves' ?