In 2D, it's pretty simple: make a mask that contains a 90-degree angle. Double-click on it to bring up the Free Transform tool to tweak the mask's rotation & scale. When you're happy, the Mask Path becomes the Brush Position for Write-On.
Oops! Forgot to mention that the mask & the tweaks should take place on a solid in the same dimensions & pixel aspect ratio as the comp. You can fool with the solid's opacity to make the mask visisble over other layers for reference. Hopefully, your comp is in square pixels, because it's easier in the long run.
KGAN (CBS) & KFXA (Fox) Cedar Rapids, IA