Training Spheres for Grip-Strength

I designed training spheres for grip strength training.
you can download stl files for speres with 80mm diameter and an scad file, where you can change the diameter to your needs.
I cut the spheres in half for better/easier printing. Just glue the two parts together when printed.

To mount the spheres, you net about 1m 8mm climbing cord.
Just put the cord through the hole, tie a fishermans knot on one side and a double bight figure eight on the other side.