While waiting on the bulk of my copper I was able to get my hands on a 2mm Plate.
Good enough for one of the caps on the waterblock.
Since I will be hand milling this block I came up with a good way of laying down a template direclty on the copper.
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First the copper is sprayed with Vinyl dye
(As it was what I had lying around)
It i will also come off very easy later.
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I then made an exact 1:1 image of the template area.
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And then used a laser etching machine to etch away the paint.
(A the machine can't etch into the copper but can burn the paint away)
The same technique could be done by simply printing a tempate on thick paper, then spray painting the tempate onto the copper.
In reality the template is only good for the edges, as the holes will be drilled though later all at once, to make sure they are aligned.
But this will help with the actual channeling later.