JQCF1,
I think you may have a hard time with this one, I just looked up the STC, it is an approval for a camera hole in a Cessna 421.
Volpar got taken over by the Gaylord corporation A Swiss company.
I would suggest finding a 421 that has been modified and seeing if they have the drawings in their paperwork.And if they, will, let you have a look at them.
B.E.