I love my Loc-Bloc, yes, it was pricey, but it works so well and you trim a stack of HST so fast.
see it here
http://www.blocloc.com/blocloc.htm
It has a grove in the plastic that goes over the seam so the template doesnt rock or slip, the edge catches the line so it is right down the middle. You cut one side, slide to the other end, cut and you are done.