This Mathdoku Editor allows you to create your own Mathdokus.
Clicking on the purple cog in the top right corner allows you to choose the grid size.
If you are not familiar with the Mathdoku Editor, you may like to start by watching this introductory video.
Once you have created your own Mathdoku, click on the 'Share' button and paste the code that you're given on the end of the following url: https://nrich.maths.org/mathdoku/main.html?code=
Now check whether your Mathdoku can be solved and has only one solution. Then you'll be ready to challenge someone else to solve it!