acorns
Hello there,
You could use a burr cutter in a Dremel type tool, if you already have one. Another option is to hand carve them in with a carving tool, carver's knife, Etc. Both of these would be carried out off the lathe of course.This way you only need one small, cheap, tool.
I have used an old Victorian pastry cutter - the cutter is very good quality steel and resembles the texturing cutters on the sorby tool - with the lathe on a very slow speed and eye protection worn ALWAYS.
You could also use a small wood rasp and sand back afterwards.
Hope this helps
Andy