Commercially available nuts never fit well and need additional work to make them work. It's par for the course that you will have to fit it to the nut channel well so that the ends are completely down and likely remove (or add) material to the ends so that it fits well width wise too.
With this said we replace lots of nuts every year and we won't use a commercially available nut even if the client brings one in and requests same. We are so very committed, or should be......
to bone nuts and saddles that we prefer to make them from scratch.As such and with your back ground, Gary you might enjoy fashioning a nut from bone from scratch. I would suggest however that you go to a local music store and buy or ask them to give you a plastic nut that is defective a commercially available nut that you can use as a rough guide for the curves, spacing, etc. when fashioning your nut from scratch. Corian is an excellent practice material.
You can slot for the strings with other files other than dedicated nut slot files but the results are never very good at least to those of us who don't like V shaped slots but it can be done.
FYI nut making is a bit of an art into itself. Some schools will tell their students that they need to make around 100 nuts before they will be very good at it. Of course everyone learns at their own rate. Also the nut is absolutely critical to a decent set-up so that the guitar plays well. Nut slot depth, the angle so that you don't get that sitar sound, all come into play with a well crafted, good working nut.