I have repaired a couple of broken mounts. I found a 2 part plastic repair kit at Lowes. It is a Epoxy made specifically for plastic, made by Loctite. Main thing I found is follow directions to a T and it holds very well. Still have one I repaired 4/5 years ago. I used three coats on it. First to get it stuck together then 2 more to expand and reinforce it.