As you can see in the photos, I've taken the edges down to the same level as the background. Mostly this was for ease of carving because I could get a knife in along the side and slide it down, smoothing things over, but after broadly detailing the rest it was looking really unfinished to me.
At this point it was impractical to drop the entire background down a further 1-2mm just to accentuate the border, so as a compromise I incised a moderately deep line where I wanted my border to be and then tapered the background down into it. This has the effect of causing a shadowed line and optically imitating a border that stands proud of the background while in reality being the same height. Ж