I’ve had a hard time with it too.
And I managed to tackle in non-so-satisfying way.
In my controller, I have this:

socket_prefix = get_socket_prefix(Application.get_env(:vinculi_web, :env))    
socket_host = get_socket_host(conn, Application.get_env(:vinculi_web, :env))
socket_url = socket_prefix <> "://#{socket_host}" <> "/socket/websocket"
render conn, "explore.html", layout: {VinculiWeb.LayoutView, "app_light.html"},
                                     node_uuid: node_uuid,
                                     node_labels: node_labels,
                                     server_url: server_url,
                                     socket_url: socket_url

with the following helpers:

def get_socket_prefix(:prod), do: "wss"
def get_socket_prefix(_), do: "ws"
def get_socket_host(conn, :prod), do: conn.host
def get_socket_host(conn, _), do: "#{conn.host}:#{conn.port}"

And I didn’t add any check_origin.

Again, this is not an elegant solution but it works (for me, at least) and don’t require tricks before pushing to production.
Hope it helps.