If you can seal it up better (as you say you plan to) to make it more air tight, this will help.
Other than that, what I do in my offset smoker when cooking only with charcoal, is fill up the fire box with charcoal and either light only a few coals on the top and closest to the cooking chamber or leave space there and put in lit coals from a charcoal chimney.
The point is to have lit coals farthest from the air intake (the firebox door) so that the air doesn't cause the lit coals to light more unlit coals quicker. Sort of a "sideways minion method". Ideally, you only want the lit coals to light the adjacent unlit coals a little at a time. This will make for a longer, more sustainable burn.
Then, regulate your air flow into and out of the smoker to allow ONLY enough air to fuel the coals to keep the temp you're looking for. Once you overshoot and the cooker temps start climbing, it's pretty hard to get it back.
More air = hotter coals = higher temps
Once you get close to your desired pit temp, close down the intake vent at the firebox door as much as possible without having the pit temp drop. Manage it that way going forward.
Fire management is KEY. It's also an ART.
18.5", 22.5" OTS
Large BGE (chambersuac's)
The Fiery Furnace (chambersuac's)
Big JT's Smokin' BBQ Competition Team