MIMF

The nut I have for the guitar I am building is about two playing cards thinner than the slot. What's the best way to fix this?

Stick in 2 playing card thick pieces of paper. You can leave it loose or glue it in.

If you want it to not show, buy a piece of bone from the pet shop, and cut a new nut that is a little thicker. You can also make nuts out of wood (generally the harder the better) corian, and other various materials.

I personally think bone is best for a guitar.

You could also fill in the gap between the nut and the headstock with matching-wood

+1 to adding matching timber - This one was done like that on purpose.

I'd just make a new nut, throw the one that doesn't fit in the drawer and hope something comes along that it will fit.

All of these suggestions are valid. It depends on how fussy you are about appearances and how creative you are with things like wooden infills. You could even compromise. Fill it with strips of cards or veneer strips and play it that way until a new, thicker nut blank arrives in the mail. I'm serious.

Patrick. Okay, thanks for the suggestion.

I always do what John suggested, whenever I file the slots too low, the nut too small, whatever. It doesn't take that long to make a new one, once you get into the habit...

Best to make a new one. Keep the existing one and make the nut slot on your next instrument a little narrower