At first, I thought the molded hand grip might be just the ticket....

Consider that ski pole handles have, in the past a mold so the fingers and thumb could be supported underneath.

Some knives have finger notches to reduce slipping.
Some pistols...

Bicycle grips come in a molded form to allow more surface area on the palm of your hand. Motorcycle grips have offered grips to reduce slipping and fatigue.

But none of those applications is requiring you to pull on the handle. Exercise machine handles allow you to wrap your fingers all the way around and distribute the weight of the pressure across the enitre length of all fingers.

Since the round handles fit, probably the easiest indicator to install is a simple white line the length of the handle and place it, so that it extends past your hand and near the crotch of your thumb. If you ever grips a golf club, its pretty much the same procedure.

I guess you could also resort to painting your hands and grabbing the handles.