Use a large piece of cardboard to create a template. Measure our the rough measurements. Cut with a knife. If you cut too much you can just duct tape a piece of cardboard onto your original piece and continue. Continue to hack it up/and or replace until you get it the shape you want.
Transfer the outline of your template to your deck wood. Cut just outside the line with a saw and then sand the edges to the line. Fit it to the boat and sand where needed. Obviously you want to make it just a little too big at first and then sand a bit at a time to fit it into place.
Randy's advice is spot-on. This is exactly how I fit the casting deck to the hull on my 16' Tatman a few years ago. It works well, just exercise lots of patience and DON'T over sand. Remove the excess material slowly and it'll eventually find it's home... Good luck.