I came across this video (Yacht Shop Creations) while looking for Soss hinge template ideas. He's a yacht guy, so he always has chemicals that go along with that. He used APF 7 Polyester filler to fill the gaps around the Soss hinge to make a router template.
Have you tried anything like this? Different filler?
Seems like a good idea.
https://youtu.be/fyb23ixAcR8