Two thoughts,
Turbulence caused as water hits each of the drilled holes in the channels does not disapate instantly upon going back into a straight walled section. Thus you could reduce the number of holes drilled with little or no loss of performace due to turbulence. And a reduction of say 1/3 to 1/2 the number of holes drilled would also give you back some fin mass. Would also save time and improve ease of making it.
And I would use a impingement slit instead of holes. A slit should give slightly lower head loss than a series of holes of the same total open area due to reduced wall contact. Would also be much easier to cut than a set of very tiny holes, agian saving time.
Keep in mind that while this sounds good to me, I havn't tried anything of the sort. This is offered only in a attempt to help add some ideas, not from hands on experiance.
Wish I could afford a small mill.