2" x 8" x 12ft long fir plank... cut to fit. You'll actually end up with a much better fit than trying to make it from any sort of template. The only thing you could use measurements for is the location and height of the motor mounts, but there should be plenty left of your existing stringers to get that. I have pictures and description of how to fit them on here under a thread topic "making progress on new stringers" or something like that.
Hint - for removal I used a 4-1/2" angle grinder with a metal cutoff wheel, a thin grinding wheel - about 1/8" thick - cuts through fiberglass like butter. Really dusty, itchy butter. It makes a nice cut nonetheless and was much better than any of the other tools of destruction I tried. Slice the glass at the base of the stringer all the way around (about 1/4-3/8" deep) being careful not to gouge into the hull then bust them loose with a few kicks. They should come out pretty intact.