A nice 'trick' that works well is to cut a cardboard bulkhead, a bit 'undersized', tape it in place - then trace around the perimeter with a 'special' marker or pen. I take a rubber wheel, perhaps one inch diameter (hobby shop), drill out the center large enough to snuggly hold the marker or pen - and use it as a 'buffer' as I trace around the perimeter, with the wheel against the inside of the boat. I cut the marked cardboard down, use the same marker against the new edge, and draw a new cardboard template, essentially reproducing an exact 'copy' of the inside dimensions, asymmetries and all. Cut that out - make a few minor adjustments, and voila'! Perfect template.