Pete
The lumber is 1/8 X 1 1/4 clear pine, some of the guy's have just picked up cheep yard sticks and cut them up as needed, length is wing width plus about 1/4, plus the notch.
as long as you don't slip them around NO Marks but I have thought about gluing on some soft cotton, but that is easy to forget after the balance is done.
Cheers Bob T