I like the spinner nuts that newc suggested on some planes, and actual spinners on others.
If you prefer an actual spinner, here is a listing of polished ones:
http://www.maxxprod.com/mpi/mpi-11.html
For a 40 size trainer you might choose 2", 2-1/4", or 2-1/2" depending on your preference. If you know the size of the original, you can get the same size. I like the spinner to be slightly smaller than the width of the cheeks that are beside the engine.