Hatch gasket material
Also have a Lord Nelson 41 with cast bronze hatches (1982). Replaced the gaskets and replaced/rebedded the acrylic about 5 yrs ago in Florida. Used standard 5/8 D gasket material and had everything installed by Lighthouse Marine in Ft. Lauderdale. They did an excellent job. Used a special sealant for window glazing, not polyurethane or polysulfide. I also use Dow Corning silicone valve lubricant to coat the seal surfaces and reapply every couple of months. Solved all leak problems. Do you have a source for portlight rubber?