Judging from the photo it looks to me like you could just add binding to the fingerboard and it will look fine. Welcome to the forum.
HI John, how narrow (dimensionally) is the neck width at the nut. This dimension is not set in stone and a half mm here and there is from experience nothing to get too excited about. Any number of makers will disagree but most of my clients can pick neck profiles and widths but seldom worry about really fine details - and .008 is nothing.
Just flatten the fingerboard edge along the whole side equally on both sides and adjust your nut spacing if required. For example, we run 3 standard widths for electrics, 41.5, 42.5 and 43.5 and that's sufficent to cover 99% of players. The acoustic world tends to stick with particular specs out of tradition (nothing wrong with that) but it's no big thing really and custom makers adjust width every day as requested.
If you are worried about the feel of the neck at the cowboy end just leave a little extra in the depth dimension at the first fret to create a similar distance around the whole neck (you can do the maths, it's a very minor change in depth that gives a major change in circumference) . This gives the player the same positioning on the fingerboard facing even with a slightly narrower neck.
Note: think about using a flat bar edge or surface plate with Stikit (or similar sand paper/abrasive) to finish your fingerboard edge so its dead straight. Do this after fretting the neck with zero relief dialed in to get a dead straight finish from fret one to the end. We take off the fret ends and a bit of the fingerboard edge in this manner before we finish the fret ends and roll the fingerboard edge.
If you are fretting over the body with the neck already on use a bar edge with abrasive to get this thing straight. Either way, don't worry way at one area with a small sanding block - it'll show up when sighting the neck.
Hopefully I got on topic with this - if not I'm sure more knowledgeable compatriots will jump in for you, Welcome aboard,