Seriousely? A ruler? If a friggin' ruler is impossible to use , how about this thing, which is basic knowledge here taught to 13 year olds? I don't know what the English word for it is, but we call it geometry triangle.
The longest edge of the right triangle is a ruler, with the zero at the center. Alongside the other two sides you van easily make angles between 0 and 180 degrees. The lines in the center are useful for making parallel lines.
This way you can make perfect shapes using just one small thing. I use it for geometry math problems.