When cutting miters on small strips of molding or glass stop, a power miter saw is really overkill. It tends to splinter the wood and can turn those tiny pieces into dangerous projectiles. To avoid this problem, I made my own small-piece miter box and used a hand saw to cut the miters like you see above.
The miter box is simply a 1"-thick piece of scrap lumber with a groove cut down the center to hold the molding strip. The groove is a bit deeper than the thickness of the strip. This way, the strip rests below the surface of the box, keeping the saw aligned as you begin cutting.
The box has two opposing 45° kerfs to cut both ends of the strip. To make these kerfs, mark their locations with a combination square. Then tape a mitered guide block (also made from a piece of scrap lumber) on the cut line with double-sided tape and cut the kerfs with a pull saw, as shown above. When cutting the miters, the idea is to end up with a strip slightly longer (about 1/32") than needed. You can then use a block plane or sanding block to fine-tune them to final size. Since this miter box is so simple and inexpensive to build, it’s easy to make several customized boxes as you need them for different sizes of glass stop and molding.