Slick now has proper 404 support.
Out of the box, Sightly doesn't seem to like outputting 404 content types in the same way as JSP. So it requires a Java class to set the content type manually. This is currently being done with NotFound.java.
By using this class, we're also able to set a proper Status Code of 404, something we wouldn't get if we used the OOTB JSP implementation.
Anyway, check out the new 404.