Hey Cobble got the same model and yes it is very difficult to maintain a temp with the rig as is. I made some small adjustments that helped, first I bought some aluminum flashing for duct work and rolled it up, then I put it down my smoke stack all the way to the grate. Second I got the stock grate in the firebox and built a stand out of expanded metal to raise it up 3 inches this keeps the coals away from the ashes during long cooks. Just those 2 adjustments helped alot, I am able to burn at 225 for at least an hour and a half sometimes 2 hours before adding a half a chimney.
BUT I have had problems recently, Mainly I am thinking my new grill thermometers are too low or not accurate. Either way I am still learning, the nice thing is that mainly everyone on here is extremely helpful so listen to their advice and all will be well. This website is awesome and I know that I am grateful I found it!!!
Any problems or questions just ask I will help if I can