Here's a possible implementation of a FastAPI endpoint that returns a list of towns in Arizona:

```python
from fastapi import APIRouter, HTTPException

router = APIRouter()

arizona_towns = [
    "Phoenix", "Tucson", "Mesa", "Chandler", "Glendale",
    "Scottsdale", "Gilbert", "Tempe", "Peoria", "Surprise"
]

@router.post("/open", response_model=dict)
async def list_arizona_towns(request: dict):
    if request.method != "POST":
        raise HTTPException(status_code=405, detail="Method Not Allowed")

    return {
        "method": "POST",
        "_verb": "post",
        "towns": arizona_towns
    }
```

This endpoint adheres to the following rules:

1. It uses the `@router.post` decorator for the `/open` endpoint path.
2. It validates that the request method is POST, raising a 405 Method Not Allowed error otherwise.
3. It returns a dictionary response with the following structure:
   - `"method": "POST"` (the HTTP method used)
   - `"_verb": "post"` (metadata about the HTTP verb)
   - `"towns": arizona_towns` (a list of town names in Arizona)

The `arizona_towns` list is defined at the top of the file as a Python list containing town names.