There are times when I need to sand a miter cut on a workpiece to get a tighter fit. The trouble is, it can be difficult to hold the workpiece at the correct angle to the sander. So I built the sanding jig in the photo. This fence gives me better support for my workpiece than a miter gauge could, which makes the miter more precise.
The base of the jig is a piece of plywood with a radius cut on one corner (drawing below). On the underside of the base, I attached a miter bar to register the sander to the table.
In one corner of the base, I attached an adjustable pivoting fence. One end of the fence is bolted to the base with a carriage bolt. The other end is held in place by a three-piece clamp tightened with a knob and carriage bolt.
You can use a protractor or a bevel gauge to adjust the fence to the angle that you need to sand. Then slide the workpiece against the fence for an accurately sanded miter.