Don’t bother trying to make something to hold the nut in place. It’s fine as it is. Millions of guitars, electric and acoustic, have nuts just like this. It’s a lot wider than a Fender nut, which makes it far more stable, so it doesn't need a groove to sit in.
Just stick it back with some wood glue e.g. Titebond. Though as it’s off I’d take the opportunity to replace the kit plastic nut with a bone or Tusq one. You can use a couple of drops of CA if you want to, but don’t use much of that at all otherwise you’ll never get it off again. Wood glue is easiest as you can move the nut for a short while when the glue is drying in order to get it central and not poking out of one end of the slot. And you just wipe off any excess glue with a damp cloth (which you can’t do with CA). I use a small artists paintbrush to spread the glue thinly over both the nut and the neck before fitting the nut.
I’d wait to glue it back on until you’re ready to string up. Just put some masking tape over the nut area when applying any finish. If you don’t want to file the nut slots to adjust the string height at the nut, then you’ll want to sand the bottom of the nut to do so, which means it needs to stay unglued. You’ll find it will happily stay in place under string pressure alone. The glue just stops it falling off when you remove the strings. I’d have the strings in place, but slack, before glueing the nut in (sliding it under the strings) so you can then tension the strings to clamp the nut in place whilst the glue dries. Leave it alone for a few hours before you tune up properly.
Last edited by Simon Barden; 05-09-2022 at 02:52 PM.