In this YouTube video, the presenter gives instructions on how to build Air Force shadowboxes.
He begins with dimensions. A standard military shadowbox on a wall is no more than 2.5 inches wide, so you should start with a 3-inch board down to that size.
You should cut the first pieces of the box, which should be 24 inches at the sides. Cut the top and bottom pieces which measure out at 18 inches. This will give your box a traditional rectangular look.
One thing you can do to make sure that your boards are exact is to lay them together and shave off a fraction of the first while cutting the second.
You will then need to place your table saw at 45 degrees to cut your corners. Cut the top and bottom boards together. Then flip and cut another side. You must then cut relief for the backplate.
You should then lay pieces out and check all cuts and corners for a tight fit. Afterward, you should cut the groove for the plexiglass. You should then join the top and side pieces but not the bottom piece until you insert the plexiglass.
You must then cut the backplate. When you have finished this, you will be ready to cut and install the plexiglass.