I need to replace some baseboard trim in my son's house a day's drive from my workshop. I do not have a miter box, but I do have a pretty good circular saw.
The photo shows the miter box I made for my circular saw. In the photo I am testing it with some 1 x 2 furring strip. In use the saw slides across a table with the saw shoe against a fence. See the next step for a Google Sketch-Up view of the miter box.
Step 1: An overview image
The miter box is an inverted "U" made from 1/2 inch plywood. The wider section toward the upper left of the graphic is the table on which the saw slides. The narrower piece on it is the fence to guide the circular saw. The hole in the side was conceived as a place to insert a "C" clamp, but is too close to the saw. It works well for holding the molding while clamping it in place from the end of the miter box near the lower right corner of the graphic.
1/2 inch plywood
Step 2: First steps
I made two pieces of plywood cut to 5 x 24 inches. In the photo a third piece of plywood 5 x 12 inches is being glued to one of the pieces 5 x 24 inches. The finish nails hold the pieces in place until the glue can dry. The other piece 5 x 24 will be glued and nailed in place to form the "U" box.
Step 3: Add the table
Cut and glue a piece of plywood 12 x 12 inches over the open portion of the "U" box. This will be the table on which the saw moves.