I'm not sure about duraboard, but INSBOARD works great. Get it from High Temp Tools:
http://www.hightemptools.com/inswool.html
It is the same stuff many commercial forges are made from. It is also the easiest way to make end caps and "doors" for tube forges.
To make the end caps, cut two circles of 1" thick Insboard to fit the forge shell. Cut the holes in them for the ports. Put 2" Inswool liner in the forge, leaving 1" of free space on each end for the insboard ends. Treat the wool with rigidizer and let dry. Give it a coat of thin satanite, including the end surfaces. Put a piece of saran wrap over the end caps, and set them in place against the wet satanite. Use duct tape to temporarily hold them in place while the satanite dries. When dry, remove the end caps, discard the saran wrap, and finish coating the liner by building up thin layers of satanite, wiping any excess off the ends where the caps will be seated later. On the last coat of satanite, give the ends of the wool a coat, too, and set the insboard caps in place. Tape them tight and let the satanite cure well. Before the satanite hardens, run your finger around the inside joint where the cap meets the liner, to make a smooth fillet around the inside joint. Let dry for a few days.
Install some sort of end clamps, or a full metal end cover, then cure the satanite with a low firing.
Finally, give the inside a coat of ITC100.
Slowly fire the forge in steps from low to full heat over a period of one or two hours to completely cure the refractory.
This will give you a very robust forge with insulated ends.
