<p>I want to show a chromeless modal window with a close button in the upper right corner.
Is this possible?</p>

You'll pretty much have to roll your own Close button, but you can hide the window chrome completely using the WindowStyle attribute, like this:
```
```
<Window WindowStyle="None">
```
```
That will still have a resize border. If you want to make the window non-resizable then add ResizeMode="NoResize" to the declaration.