It's a challenge gluing miter joints. There just isn't an easy way to clamp them together. The store-bought clamps I've tried in the past didn't always pull the joint together. So I came up with a shop-built corner clamp that uses wedges to push (or press) the mitered pieces together.

Looking at the photo and drawing, you can see that this corner clamp uses a piece of plywood for a base. Then a square block and two cleats are glued and screwed on top. What's important here is that the inside corner of the square block is exactly 90°. Then align the inside edge of each cleat parallel with the inside edges of the block. A pair of wedges sized to fit between the workpiece and the cleat do all the work.

They apply pressure in two directions at the same time. When the inside wedge (the one contacting the mitered piece) is tapped forward, it pushes the joint tighter together and firmly holds the piece in place while the glue dries.