Opening and quick try of the Clamp ($20 from Amazon). I’m using it for welding 1″ aluminum tubes and it does that well.
For larger/heavier peices I’m not too sure. The maximum size T you could weld is 1 3/4″, for a right angle you could fit around 2 7/8″.
The threaded nut is free floating, I didn’t think the “head” was at first, but it was just stiff and will move from side to side also.