This corner clamp is easy to make with table saw.
Wooden wedges clamp the material and keep it square.
Four of these clamps, resting on a flat surface, will keep the project square and level.
They are now included in my clamping arsenal.
Video: https://youtu.be/BfYqk_x_aDo
Corner Clamp 3.jpgCorner Clamp 4.jpgCorner Clamp 2.jpg