Store-bought miter boxes are made to fit saws with kerfs of varying size. But this one-size-fits-all solution can lead to flawed cuts. Instead, you can build your own miter box using some 1 X 3 ...

How To Make Your Own (and Thankfully Discreet) ... Miter cut four pieces of ¼-inch thick and ½-inch wide poplar that form the frame for the vent, ... How To Make a Box.