I keep several sizes close by on my bench for various sized openings.
Simple pieces make-up this gauge. Two thin strips of wood and a spring binder clip.
Below, the gauge is being used to measure the dimensions for a thin plywood insert to conceal the pocket screw construction.
Below, you can see one end of the gauge registered against one edge of the dimension being measured.
Below is the final fit of the plywood insert.
Finis for now!