Practice run with the router. First step, trace out the cavity. Then drill as much material as you can away with a forstner bit.
Set the depth of the router blade. I went about 2/3 as deep as I need the pocket. This is pine, though, so when I do the real thing, I'll probably do it in quarters. 20 mm is appx the final depth I'll want.
Here's the little vid I promised :
Then I drilled one spot to the 20mm, set the router depth accordingly, and started there. Taking it slowly, I got it to this :
I'll finish the edges with a chisel or gouge on the real thing.