First things first, most modders are now on D4N, so you really should post any modding questions there as most here will go by unnoticed.
http://forums.duke4.net/index.php?showforum=26
Secondly, may I ask why you are using Dos duke for a mod. I could understand trying to keep to the roots of the game, but if it comes to playing a mod I'm going to be using EDuke32 and if I need Dos box to run it I'm not likely to bother.
You do know that EDuke32 does have a software render which displays the game just how dos duke would?