I managed to create a radioactive symbol for the snail! It took me some time, trying to get going with the shell stitch. Until I finally realized that I was making it too complicated. All I needed were single stitches and half double stitches. It now looks like this:
I know that there are still some bulges at the edges, but that’s because there is a slight difference in wool: the yellow wool I used was slightly thicker than the brown one. If you use the same wool type for both colours (which is, of course, always advisable), it should work out fine.
Now I want to draw up a graphic pattern for this sign. I guess that will be my next challenge…