The design of this stop block has an added bonus... Since clamping a stop block to the miter saw fence is difficult on shorter workpieces pieces, the stop is long and attached to a plywood auxiliary fence.
The stop is pretty simple to make: two pieces connected by a couple bolts for adjustability. The mounting block is L-shaped, with one hole for a hex bolt to fine-tune the stop block and another for a carriage bolt to lock it in place. The carriage bolt is counterbored, while the hex bolt threads into a T-nut.
The stop block is a long piece with a cut-out for the mounting block at one end and a chamfer for dust relief at the other end. A recessed rare-earth magnet keeps the stop block attached to the hex bolt as it moves. A slot along the side accepts the carriage bolt. A washer and knob thread overtop, and tightening the knob locks the stop block in place for a cut.