Ideally, when making the skull, you would have made a wire loop and inserted it while the foam was wet. So, a screw eye is the next best option. https://www.walmart.com/ip/Fabory-1W...l13=&veh= sem
They can be got in many sizes, a 1/4" wide top is probably comfortable.
EDIT: better yet, put two screw eye about an inch apart. two areas of attachment will make it wear more stable.
You might have to make the hole and then glue the screw eye in place, some foams strip out way too easily.