Yes ,it needs a bit of flattening ..Not much though ...
Try to find a flat (but as flat as possible ) thing ,about the size of the COBs you want to install .
Check again with that .
Also @ john0000
Since you've finished drilling the holes ,
do not forget to smooth out the hole lips ,before sanding ...
You 've to do that ,so that the COB/holder will "sit " totally in contact with the heat sink ...
2 ways to do that :
1) Either with a razor or sharp knife ,take away (slice / cut lightly around the hole lip ) any "chips" remaining at the hole lip .
2) Lightly countersink the lip of the holes.Just lightly.
Use a drill bit double the size of the one you used to drill the hole .
( if you used 2.5 mm (for a M3 thread ) then use 5 mm for a light countersinking.
- I'm not familiar with the US drill sizes )
a ) Set the drill in motion,
b ) center it over the hole
and then c ) lightly touch the hole lip with the drill bit .
You do not want a full countersinking .
Just enough for the hole lips to be clean and smooth ,
without any "chips" from drilling .