We have a few cheap corner shelves we got from Amazon (https://www.amazon.com/ARAD-Black-Laminate-Radial-Corner/dp/B01M8Q1VLI/ref=as_li_ss_tl?ie=UTF8&linkCode=sl1&tag=meto061-20&linkId=2832617c3df82c186b733ec8d4e2e954&language=en_US). While I'm sure they work great on a nice, fresh, new, perfectly square house, they don't quite sit level on our walls. I created this brace to add to the "open" end of the shelf to level it up and hold it steady.

The hole should fit a standard drywall or wood screw, and will be hidden by the shelf once installed.