We removed the old pattern with acetone (washing quickly afterwards with water). Came off easy. Have a sign-maker make one for you to your dimensions. Just explain you want a sticker that lets you see out but not in.
I found the L450 Lagoon
window stickers on the Beneteua parts
site, and used their dimensions (423mm x 910mm), but turned out they were about 5mm short on all sides, so suggest adding 5mm all round. Easy enough to trim off. I had the guy make the sticker as a rectangle as did not know the radius of the corner, so after applying sticker use blade to curve the radius in. My fully white border to protect the adhesive
If you able to make a pattern for your sticker using brown paper or something, even better. We were not at the boat
at the time, hence the guess work