I have a lid much like that on my UDS, it is a wal-mart dome grill that I flattened the edge on to make it fit. It leaked pretty bad at first but the grease from cooks tends to seal it after time. I never really had trouble controlling my temps, the leak is so far up it is pretty much just an exhaust. I don't think that was your problem.
3 UDS's / Orion cooker / gas cabinet converted to charcoal / Weber kettle / Cowboy grill