Corner molding into a tray ceiling

Gang -
I'm finishing the last part of an installation of a tray ceiling on a porch. The 19"-wide part between the walls and the ceiling slant in at 21 degrees from the horizontal. The ceiling has molding around it. The corners where slant-meets-slant-meets-ceiling-molding need molding to cover the imperfections of earlier carpentry. I am essentially trying to fit a board onto a corner of a cube at an oblique angle to all surfaces of the cube.
Setting aside the angles for now, I can't figure out how to make the cuts. I'm essentially trying to make the hollow of a point--and anything that makes a cut to match an angle mars a different face.
Is there a power tool solution? Is this the time for hand tools? And if so, which ones how used?
I tried gouging out the back of a test board with the table saw. To get the height I needed took too much wood in length from the underside of the board and it cracked on installation. My current thought is a router table.
Any thoughts from the collected wisdom? I already know that if I'd known it was going to be this hard I should have started differently ;-)
Thanks in advance. ...best, Capt N.
--
Email to Captain snipped-for-privacy@C0X.NYET (yes, you can so figure it out) ;-]

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Instead of trying to miter the joints, can you butt them and use a coping saw to cut the mate. Hard to explain - Hope this is of some help.
Erik
(yes, you can so figure it out) ;-]

broke. ---Kilroy
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