developing a product to help with hand tremors

I am an engineering student from the US working on a project at the National University of Singapore, and I am developing a product to make it easier for people with hand tremors to unlock doors using keys. My team and I have been unable to find many people with hand tremors willing to talk to us, but we would really like to know what you would like to see in a product like this and how we can best help you.

Some of the important criteria for the product that we have identified are:
-should make key easy to grip
-guide key to keyhole easily with correct orientation
-easy to operate
-compact
Please comment if there are any other qualities that would be important to you.

Some other questions we have:
-Would you prefer an attachment to your key to be smaller or larger? Which is easier for you to grip?
-What issues do you encounter when trying to use keys?
-How do you hold/use your key?
-How much would you pay for a product like this?

Anything helps, thank you!