Most of this video is about repairing a guitar nut, but I share it for what
is shown in the first minute or so:

The hardness of Baking Soda mixed with Super Glue

https://www.youtube.com/watch?v=slCMkvEfK_U

There are other videos showing the mixture used as a gap filler, etc. and
then filed, shaped, etc.

One video notes that the mixture gets very hot, so you'll want to avoid
getting any on your skin while it cures.