I just had to rip out a contractor FUBAR for capping stair treads. I am almost finished prepping the original steps for the new risers and treads I will be installing but need to make a change in the bottom tread. The existing tread is curved on the right side and I want to square it off. I am looking for advice on building a box for the new right corner. The new riser will traverse the width of the tread, 42 inches, and then go under what was the curved part. Sorry for the trash bag over the curved part of the step in the image.