The hardest part was getting the blank absolutely flat and square on all its faces. My hand-planing skills are still fairly rudimentary; I just can't seem to master planing square. If the blank isn't square to begin with, then everything that follows will be subtly out of whack.
Once that was (eventually) achieved, the initial marking and cutting went smoothly enough, but carving out the body of the gauge so that the faces were smooth, flat, and even was trickier than I'd anticipated. I'd thought, when I started it, that I could flatten them with a router plane, and I could have, but I'd have had to make a jig to hold it and provide surrounding surfaces for the router to rest on. If I was making several, that would be worth while, but for this one I just finished the faces with a chisel.
The oak made that slightly problematic, because the grain made it difficult to see precisely what was going on with the surface, and it tended to either catch the edge of the chisel or deflect it. It would be better to make something like this out of beech, I think; absolute smoothness isn't strictly necessary on the faces for the tool to function correctly, but my perfectionist soul would prefer it.
Next Day
I made another one, this time out of a wood I can't positively identify. It may be a species of beech, or possibly pink birch, or even a pale mahogany, but I just don't know. It was originally part of a bit of furniture.
It's smaller overall than the oak one, though neither is particularly huge.
I cut it out of a longer piece, which allowed me to carve the valleys flat with a router plane with its plate resting on the thick bit in the centre and the sacrificial horns on either end. Then I cut the horns away.
As I suspected, that made getting the valleys flat and even a hundred times easier.
