Router and template! Use a band saw or jig saw or even a coping saw to cut out the shapes in MDF or something. Use a 1/2" drill bit as a template and draw on your rounded corners. And then sand or file or rasp them to shape. THEN use a flush trim router bit and go to town!
Woodworking
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An idea that came to mind.. I do have a 3D printer so I could make 120° and 72° round radius guides like these.
If you didn't have a 3D printer, you could still print a template on paper, glue that to a board, saw/rasp/file/sand to the line and then use the shaped board as a template. Double sided tape should probably be good enough to keep it in place on your actual workpiece.
Could you not just use a router pattern bit and use the actual piece as the template?
This is the way. Tape it down, route it out.
If you have a disc sander, you could make a jig that has a peg sticking out that you rotate the shape around the hole. Something that looks like:
If you don't care about being perfectly exact you could use a Shinto rasp. Very effective and cheap