I ended up with some beautiful markers which I'm very proud of, using clay, a teabag, PVA glue, and a lot of patience!
Step 0: Try out a bunch of different techniques and ideas.. you can see here I was aiming for a natural aged effect for the markers. Experimentation taught me two key things:
1) Thin clay snaps quickly
2) Water down your colours to enable you to wipe away excess completely.
3) If you are aiming for an old effect, acrylic is NOT the medium to use.
Step 4: Smooth off the rough sides very gently - the clay will still be a bit moist at this point. I used an old scalpel blade.
Step 5: After a further 24 hours you are ready to paint the markers. Boil water and soak a teabag in a little water - just enough to paint the markers. I went all over the markers once, and then concentrated painting tea into just the imprints for another 4-5 times.
Step 6: You guessed it, wait another day. The markers will be dry now, and can be painted with PVA glue or varnish to seal them. I then wrote the names of my plants on the back - here is Parsley's marker in action