I’ve made a few bone nuts from blanks now. I have an oscillating spindle sander and that makes light work of the bulk sanding. I get it almost to size on that and then use sandpaper on a flat surface to do the rest. Once you put your mind to it, it doesn’t take much longer than sanding a pre-bought nut down to size (they are rarely the right thickness or height).

Whether you can use a 43mm nut filed down at the ends on a 42mm wide neck really depends on how close the string slots are cut to the ends of the frets. There isn’t a universal standard. It’s normally about 2mm to the outer edge of the slot, but I’ve seen noticeable differences on guitars (I’ve worked on far fewer basses) from the same manufacturer. And pre-bought guitar nuts definitely have differences in the overall string spacing.

I have a small stock of replacement bone nuts of slightly different lengths and widths and heights, and even on nuts of the same length, I know the string spacing can be different. I’ve recently had to make my own nut for a Squier Tele neck where the slot was quite wide and on the only wide nut I had, the strings were slightly too near the edges once I’d taken 0.5mm off each end. It was too easy to push the low E string off the board when fretting chords. So that had to come off and a bespoke one fitted.

It’s probably best to measure the distance between the end of the nut and the string slot on an instrument you already have and like the feel of. If you can get the same distance at the ends after filing 0.5mm off, then you’ll be fine.