I should have done a few more pics but what I did was make a hole jig out of 16mm thick plate and milled it to fit inside the channels. I marked the plate as accurate as I could where the holes go and drilled a 3.5mm hole at those points. I used thick plate so I could drill buy hand and the thickness kept the drill bit vertical. With so many to do it was much quicker to do by hand rather than the drill press.
I also made a countersink tool to fit my cropper.
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