Look at picture #7. 3673 is a triangle that the screw insulators GO THROUGH. It is also THIN. If you make your gasket too thick it will cause the 3630 plate to contact the 'C' ring which makes the horn blow. If you make your own gasket, use heavy plastic, like from a motor oil bottle. You should 'feel' at least .030" air gap between the 'C' ring and 3630 plate.
Yes, these parts should be isolated from ground BUT, 12 volts DC will not shock you. I doubt your hands can feel 12 VDC. Mine can't. - Dave