Customizable model link (Onshape)
There are two versions - one with grooves with a flat bottom and one with spherical ones. Personally I recommend the models for the flat bottom one since it should be easier and cheaper to print
The model dimensions were tailored to fit comfortably in my hands.
Pictures included are a print of an older version which i've since updated for better usability
The author marked this model as their own original creation.
