Thank you everyone for your kind comments.
It is an air-cooled engine worth more than my first house, so really good baffles are worth thought and careful fabrication.
BTW, I can't say if a 1/16" or an 1/8" or any other particular dimension is "correct" for this duct. Can't even say if the left front and right rear cylinders should have the same duct size; different pressures in their respective plenum areas. I am sure an install which doesn't allow any air to a whole section of the cylinder head finning is bad, and likewise, all leaks are bad.