What type of smoker are you wanting to make? I say that because like a Weber a vertical like a UDS don't need to be thick because it doesn't need to get very hot to do the job.
Making memories.
Current Smokers: Backyard RF Offset and Hybrid RF Offset trailer rig with Cowboy cooker and fish fryer, always room for more........
Thicker means more efficient. You can do it as thin or as thick as you want and still have a usable unit but it seems 1/4 to 3/8 to be the average for an offset.
Normally 1/4'' is minimum but it is usable at 3/16''. Be sure to take your time when cutting the door out and welding the banding straps on the door. If you get it too hot it can twist out of shape and you may never get it to fit right.