First of all I masked everything with painters tape. Glued graph paper on top and sides. Marked out the design. Tried to drill an 11mm hole in the side. Quickly realised that would need pilot holes. 4.5, 7, 9 and 11mm drill bit later I had the side holes...oh, I built a stand to hold it leveled and steady under the drill. 5mm pilot holes x 7 along the length of the longitudinal hole. Used a 16mm quick drilling bit (flat with a large prong in the middle and smaller ones on each side) and followed the pilot holes one side, turning it over to finish it off. Chiseled out a few bits, dremel with sander from there. No issues with chips.
With a lack of band saw it turned out quite deep and I have a problem with strings hitting the headstock on there way to the tuners. I may have to redesign that section of that headstock leaving the truss rod cavity exposed...bugger! I’ll do my best make it acceptable.
At the end of the day this was an experiment and I’ll be happy if it works.