I need small saw miter box to cut small size soft wood. The one I have is rather big, so I have to design my own. The design make use of longer rail because my experience with miter box is the rail wall tends to be widen after a few use as it's too short.

The design is in onshape (link below), feel free to copy the design and change the dimensions.
https://cad.onshape.com/documents/8fa53421d023253e97e725bb/w/128b8797d7db67880b434e73/e/731c7e5672e772b0259c11b6