I am in a similar situation, actually have been for years.
Looking for "deep" ball torture without marks.

What I found out: The less impact sound it makes the better the ratio of pain to marks.
Riding crops are really bad, they provide a loud clack on impact, but there is no deep pain. My suggestion: Take a hand-towel, roll it up along thee long edge (so that is long) and make a knot on one end. Swinging this into the balls makes no noise, very little risk of marks, but can cause really bad pain.
But my ultimate preference is squeezing with the fingers (if the domina has short finger-nails).