A lot depends on how you want to fly your Eindecker. If you expect to do even mild aerobatics you need functional wires both top and bottom. Something of this nature http://shop.balsausa.com/category_s/141.htm
If you are going to just fly circles, do touch and goes, etc. then most any other type of material can simulate the wires. I used black fishing line. That was fine for about the first 20 flights with mild aerobatics. After that the wing started flexing and i had to stop. The plan is to add functional wires before spring.