I wanted a hatch, so that I could access the space under the fore deck. I figured out the size I needed and then spent about a week trying to work up the courage to cut the hole.
I drilled 2 holes through from underneath, right beside the each of the two beams so that I knew exactly where to cut.
I marked out the 300mm square, twice. And then set to work with my jigsaw.
I reinforced the sides of the hole that were not over beams with offcuts of sapele, epoxied underneath, and then bolted the hatch in place with a bit of silicon sealant.
