Tips to get (close to) accurate bevel angle by handsaw?
I did something similar to this in my younger years when I needed a board cut with a 45 degree angle. I marked the cut line on the edges and face of the board. Then positioned the saw to the cut line and clamped furring strips on the top and bottom of the board up against the positioned saw the length of the cut. the two furring strips acted as a double fence for the saw and the angle turned out pretty good. If your miter saw ever quite mocking you, I'd use it but I totally understand your desire to use hand tools...