Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:5249762.
Remixed to support common 1"x2". Also made a thicker design to handle the horrible drywall hack job the original builder performed. We are hiring a carpenter to make a skirt board to finish the job. The stair tread tools available at the store would've been too long to align our risers. So I made varying length versions just in case.
(2) 1/4" x 3/4" Hex head bolts (2) 1/4" x 1 1/4" Machine screw flat (countersunk into 1x2) (4) 1/4" Wing nuts (4) 1/4" Flat washers
Common 1x2 board cut to length. About 3" shorter (if I'm remembering correctly) than the width length of the riser/stair between the walls.
Adjust the length, tighten the wing nuts. Adjust the angles, tighten the wing nuts. Carefully remove the jig (so you don't know it out of position). Align the jig to the riser or tread, and transfer the angle by marking with a pencil. Cut with a sliding miter saw (mine doesn't slide :( ), or a circular saw and ripping jig (why didn't I make one of these years ago?).