Compound mitre cuts

I hope this is the correct place to post this.

I need to create a “duct” for dust collection on my mitre saw. Its a 6 in tall box that has compound mitre cuts on the corner and I am struggling. I have searched for videos on youtube and found things that are close but I can’t get it right.

I have the in side dimensions and material thickness. Top opening is 19" x 10" and the bottom opening is 11.75" x 6.75" and I am using .75 plywood.

I thought I had the angles at 30 and 15 but when I cut them with 45 degree in the corner the angles in the corner is too steep.

if there is a tutorial that would help I would like to learn this for future projects.

Looks pretty close. I am confused by the 45.6 vs 49.6

I feel like the angles on the ends will be different from the angles on the long sides.

inside dimensions. I have a plastic piece for the short end that connects to my dust collection.

There’s no need to model the panels with thickness to merely find the miter angles and lengths.
Simply model the exterior Faces only.

There is a (free) plugin which will yield the dihedral angle.
The miter angle = the dihedral angle / 2

Angle Between Planes v1.0 — SketchUp plugin by Clark Bremner

Read the plugin documentation and then experiment with it.

Here’s what it does.
Right click on the common edge of two faces.
The dihedral angle is immediately displayed in the context menu.

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Awesome thanks

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