I have a very leaky weber lid on my UDS from taking a weed burner to it. Its still able to hold a temp at 220 for hours and hours.
I use a weber 18 inch grill and preferated metal as a basket. I fill it completely with one bag of lump and then I use a half a chimney and it never gets close to 300 degrees
BUt, if I miss the temp on the way up and dont close the vents soon enough, then it takes forever to control and can be very very frustrating.
1 Raspberry PI Heatermeter homemade temp controller UDS