I usually make a few cardboard (poster board is easier to work with) templates, each one getting closer to the perfect fit. Cut them somewhat undersized and tape on pieces of extra cardboard to make them fit the hull in each position, then after it gets messy, take it out and trace on a new piece of cardboard that now fits closer and try again.

When I removed a foam bulkhead form a factory boat, I noticed that the makers had run a saw cut/groove/(melted groove?) around the circumference of the bulkhead, in the middle of the thickness. I think the idea was to provide a bit of extra flexibility + a reservoir for the sealant ??