u/shiauface

I have one of these nozzle extenders to raise the nozzle above a boxwood. However, the extender keeps popping off, leading to a fountain of water in the coverage area. I've swapped the extender out a couple of times and it keeps happening. It's already a 12" nozzle and digging it out would require digging past two sets of pipes. Any recommendations on how to keep these extenders from popping off?

Two photos. One from the outside and one from inside. If I trim the outside more than a little, I hit bald spots. Why and how can I fix?