I would not use any adjustable type elbows, unless they are metalbestos, triple wall or the like. Remember that this will reach upwards of 1000 degrees.
If you can follow this... use a piece of 18 gauge sheetmetal...cut it out to the shape of the top of the fire box. Make sure that you leave some tabs to attach it. Install it in the firebox with sheet metal screws, install your insert. Now plumb a line up the flue of the insert. Measure the height that the flue pipe should be( remember the distance to fit in the stove +1") Remove the insert, remove the sheetmatal. Cut out the flue opening in the sheetmetal, 1/2" smaller than the flue pipe. Cut every 1/2" back to the line for the flue pipe. Bend these tabs towards the stove. Make sure the flue fits in the hole.
Install the shield, push the flue pipe in the hole, install the insert, pull the flue down to the stove. Get a large stainless steel clamp and put around the tabs to tighten to the flue pipe. I could have installed this in the time it has taken to try to explain it.