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Marked out holes in fuse and used a hole saw to get close, then filed for a nice fit.
First 4-40 countersunk hole made a mistake by trying to counterbore 3 BID by itself.
The remaining 5 holes went well because made sure Ultimate ventilator ring was held in position and used as a guide/backing.
It was necessary to give a slight counterbore on aluminium ring to allow screw to be fully seated on fuse.
Y9-07-16