I’m making my Max with a cloth body because I only have the head and limbs. Since I don’t have his body he needed a neck. I pondered this for a long time. This is what I came up with.
The white piece is a prescription bottle cap. The part that screws onto the bottle is the perfect size for his neck opening. I found the red thing at the hardware store. I don’t know what it’s really used for but it worked for my purposes. It’s slightly flexible but almost hard so I had to soak it in boiling water to stretch it enough to get it over the cap. It was a bit of an effort but I got in on there. Not too shabby, huh?