I know you are trying but that photo and ruler is not enough. Making a specific scale part to fit in a specific space is not that simple unless accuracy is not an issue.
If I were making that piece I would need the actual piece to then use my digital micrometer to ascertain the various dimensions in hundredths or thousands of an inch.
then I could accurately design the piece as a 3D model graphically.
AT THAT POINT, I can export the 3D model into a format for my 3D printer.
Otherwise, the model I make would be totally useless.