Hello.

I needed to make tile textures for a mod, so I have written this simple GIMP Python script to automate it.
You can download it here:
https://gist.github.com/piotrekli/cf2a88d5ca5d05c8b2fb6a52d716232d

Usage:

Create a "stencil" image consisting of 5 layers with the shapes of your borders ...