Head Meets Neck Question Part Deux

I ordered the Wondery Wonder lovers Delia head to put on the TBLeague S24-A body but it has a neck connection I've never seen the likes of before (see pic, it's a little plastic hollow compartment with a really big opening). I'm presuming I'll have to use some sort of filler around the little TBLeague steel neck to create a stable connection, but what is the best substance to use?