I screwed up the first one, but not so bad I couldn't learn from it and recover.
I used a table saw!
And a thing called A Shop Smith Mark 5.
First I tried to drill through the two piece block that was already bolted together. That didn't go straight at all!
So I dropped the blade really low in my table saw and cut a groove in each half. I moved the fence and changed the blade height to cut out several passes. I removed about as much material as possible with saw cuts. Then I bolted the pieces back together and started drilling with bits that were just a tiny bit bigger than the channel I cut. I worked my way up to 12mm.
I also polished the surfaces that fit the TEC with wet sand paper on a piece of plate glass; 180, 320, 400, 600, 1000, 1500 grit.
James.