Just to add in again. Blu is right. Don't bother with building a charcoal basket. It blocks airflow and will make it harder to keep temp. Not to say you shouldn't cook with charcoal on your offset though. You can cook with it and get great results but build a charcoal fire on the charcoal grate in the firebox and add a few chunks or splits of whatever wood you fancy.
And remember, offsets are all about draft and air flow. Adjust your temp with the intake only. Leave the exhaust open 100%. You want a good draft to move heat into the cook chamber and leaving the exhaust unrestricted achieves that.
18.5" OTS, 22.5" OTG, CB Silver(Rip), Vintage Coleman Bullet, Pit Barrel, UDS