I did a similar thing when I built some bookcases for the top of the stairs.
I used https://www.screwfix.com/p/trend-s-dar-200-digital-angle-measurer/1192k to measure the angles on each side (don't assume they will be the same) and my trusty Titan (now McAllister) track saw to cut them. The thing about the track is that it gives you a very neat edge by preventing breakout.
A little finessing with hand plane and a detail sander and it fit snugly.
I did a similar thing when I built some bookcases for the top of the stairs.
I used https://www.screwfix.com/p/trend-s-dar-200-digital-angle-measurer/1192k to measure the angles on each side (don't assume they will be the same) and my trusty Titan (now McAllister) track saw to cut them. The thing about the track is that it gives you a very neat edge by preventing breakout.
A little finessing with hand plane and a detail sander and it fit snugly.