Need method to open/close a 4" vertical tube to adjust spillover level

[cavitate]

I have a 4" vertical thin wall aluminum spillover tube, I want the spill over tube to have two adjustments besides the full height. There is not a lot of room to make these adjustments, but probably just enough depending on solution. I have include some relevant info in the drawing. My first idea is to have internally threaded inserts heli-arced around the circumference of the 4" tube, and put threaded plugs in the holes.
Thanks, Mike

 

[cavitate]

I have added one idea to the right side of the original drawing. A sliding sleeve.
Thanks for your input, Mikek