To further Bruce's suggestion...
A while back, Lee Valley created a saw guide that has embedded magnets to adhere to a saw blade covered with a thin piece of teflon, to allows the saw to slide against the magnets easily:
http://www.leevalley.com/en/Wood/pag...18&cat=1,42884
You could easily rig up something similar to a hardwood block, as Bruce suggested, but with the addition of drilling a few holes for embedded magnets and putting a veneer of teflon over it. That would keep the saw cut square across the board, square to the surface of the board - a potential difficulty with un-aided hand sawing - and prevent the saw from wandering out of the kerf.