Here's a fixture that my dad told me about a long time ago. It's a good alternative if you don't have a miter box at hand. As shown in the drawing, my bench hook is made of a 6-in, by 10-in, plywood base, with 1x cleats screwed to the top and bottom on opposite sides.
In use, the bottom cleat hooks the edge of the workbench, and the heel of your hand holds the stock against the top cleat. A pencil line squared from the top cleat guides the saw for square cuts.
Don Fenton, Brooktondale, NY