Instead of using using an accessory to place the end cap, use a different frame with a cap built in. 'Modular Frame 15' should do what you want.
If you need the external cap to have a different material to the interior frame, then Laura and myself both posted identical custom versions of that object. Either will work. These two threads should explain how to add it to your library -
http://archicad-talk.graphisoft.com/viewtopic.php?t=30641
http://archicad-talk.graphisoft.com/viewtopic.php?p=159714
Hope that helps!