Progression of templates for the 3/8 casted acrylic windows I made for the big non opening ports on Sea Wind. There should be a book about how many things cardboard can be used for haha. From left to right was the first tracing of the window opening, then a rough cardboard template in the shape of the actual window size. After that I transferred the shape onto some 1/8” clear plexiglass and then into 3/4” MDF which is what I used as the stop for the flush trim router bit. Each template I refined the shape more into what I wanted it to be. 1 1/2” of mating surface should be adequate for no leaks! The old windows had barely a half inch of sealing surface.