I found a site that has lots of corner shelf designs & how to make them. I really like this idea as it requires minimal effort when it comes to actually "building" them. You could probably get some place like Home Depot or Lowes to actually cut the wood for you in the lengths you need & then all you need is the hardware (brackets) and some screws & stain or paint to finish them.
https://ahomewest.wordpress.com/2012...orner-shelves/
Here's the site that has other ideas. Just scroll down until you see the words "view plan" and it will take you to the instructions. The one above is #4 (Stacked Corner Shelves). Hope this helps you.
https://donpedrobrooklyn.com/diy-corner-shelves/