Embed 3rd Party Server Control into my Custom Web Server Control

[Tim]

I am trying to place a 3rd party server control (Infragistics
UltraWebNavigator) into my custom web server control.

I am rendering the html needed for the control in my simple server
control. In the web form which uses my server control, I have the
directive:
<%@ Register TagPrefix="ignav"
Namespace="Infragistics.WebUI.UltraWebNavigator"
Assembly="Infragistics.WebUI.UltraWebNavigator.v3.1" %>

I've made an example that is as bear bones as possible, but the
UltraWebNavigator control never renders.

Anyone have any idea what I need to do?