If you just bought a manual miter box, a simple modification will extend its useful service life. Insert and fasten a piece of 1x stock to the inside bottom of the box. The saw blade will then cut into the false bottom, without damaging the miter box itself. Occasionally move the wood back and forth, or turn it over, to spread out the wear. When badly cut up, just replace the piece.
When tackling a home improvement project, always keep an open mind. There are hundreds of design opportunities to accomplish your goals.