I've made an odd corner before, I cut the angle for one side with my 12" mitersaw. Then I nailed two 3/4 x 3/4 strips on a work table board, at the angle I needed. I sat the cut piece on one side against the strip, then I fitted the other side to it with a duct seaming tool and a pair of snips.
The mitersaw, made a good finish cut and the other side of the miter was cut and bent to fit inside it.
It takes time...
You can do it.
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