Welcome to SMF... I am kind of new here myself. I have a CG DUO with a SFB. I was unable to get a long run for ribs/brisket/Boston butt, due to the ash building up in the SFB and restricting the airflow. I took out the stock charcoal grate and replaced it with a piece of expanded metal.
There were two holes left from removing the stock grate support. Put a bolt through the hole, with a nut, and use that to "clamp down" the SFB lid as it did not seal well with the SFB.
Extending the stack down to near grate level in the main chamber will cause the heat and smoke to flow around the food, not up to the top of the lid, across the lid and out the stack. I used a piece of 3" hard dryer duct and a hose clamp. This required that I take out the rack, but the rack got in the was of smoking larger chunks of meat. I put a hook on my porch rail and hang the rack there when not in use. The nuts come off with my fingers. There is enough "seasoning" on the inside of the main chamber, that I now just slip on/off the stack extension as needed when smoking. I do not use it when grilling.
The stock thermometer are not located at grate level. That is where the food is located, so the thermometers should be there too. I purchased two thermometers on Amazon.com and installed them in the lid just above grate level. Check your thermometers in boiling water to see if they have an error.
Check your smoker/SFB for any open holes and fill them with a nut and bolt, if possible. Control of air flow is what you are looking for. Some folks go as far a making a gasket to seal the main chamber, others use foil to make a disposable gasket. I just let it roll.
I use a water pan to help even out the temp in the main chamber. I set the pan on the main chamber charcoal grate and fill with about 1/2: - 1" of water. Others have made a plate to help disburse the initial blast of heat entering the main chamber. Some turn the charcoal grate up-side-down, hang it from the hooks and raise it up to channel the smoke heat into the main chamber. Lining the bottom main chamber and grate (if grilling) with foil makes it super easy to keep the the smoker clean too. Use a brass brush and oil your grates.
I hope this helps.