During the middle part of the last century, I had a mechanical drawing class, even though I had not yet reached high school. One of the skills taught in that class was the development of drawing the intersection of two bodies. I believe this question qualifies in that regard.
Just about any drawing program will work for your objective. One can use a CAD program with which one is familiar. I use Inkscape (free, multi-platform) but also LightBurn, a laser application (paid, multi-platform) but any line drawing software will do the trick.
If one has access to precise, accurate drafting equipment (T-square, drafting table, triangles), one can also do this manually.
The key factor is to have orthographic views of the model. If your model is in a contemporary 3D modeling program (Meshmixer?), you can set the views and use a screen capture to create the orthographic images.
A YouTube video presents the overall concepts and practice of drafting such a project. I used "manual method drawing cylinder intersections," even though one of your objects is not a cylinder. It would have been far too challenging to determine proper terminology for the hemi-cylindrical shape engaging the cylinder in the image provided.
The results of the search are consistent with what little I recall from so many decades past. In your project, you'd end up with a drawing of a rectangle representing the face-on view of the cylinder with an arched rectangle centered in the rectangle.
Because the result of a rectangle represents a squared view of the cylinder, there's a perspective loss, although I'm not sure I'm using the right terms.
I changed the search to "sheet metal drawing intersecting cylinders" and discovered a more comprehensive result.
Wikisource provides a series of problems and solutions similar to your objective:
Combining the above video with this Wikisource information may give you the background necessary to accomplish your goal.
If you've created the model, knowing the program in which it was created may be useful.
Additionally, Pepakura (not-free, not expensive) provides for "unfolding" an STL file and may be the fastest and easiest option, considering the amount of work required to CAD out or hand draw your project.
Another program for Windows, Ultimate Unwrap 3D runs US$50 - $60 depending on version. The home page graphics indicate that it would provide a similar result: