Hello am a new member long time griller but new to smoking I have a chargriller duo with the sidebox. I'm having trouble keeping my temps under 300 without keeping my box door cracked open so I lose smoke . any tips
See if you can run with the firebox door wide open, using small sticks of hardwood.
You are seeking the pale blue smoke (almost clear really) and you won't get there with any restriction on the air intake.
Like above poster noted, smaller fire and monitor it like you would a sick baby.
In my duo, I will fill the chimney starter about half way with briquets and ignite for about 5 minutes on the side burner. I will turn it off for a bit then place on the burning rack, along with some solid clumps of hardwood. These can be finicky for sure, that's why I bought a pellet smoker..., but I will usually play with the box vent and the chimney vent to control things. Also, get a spray bottle with water. I find a periodic spraying into the box vent helps to settle things down too.