Hop over to your local home improvement center and check out the moldings. You can make any size with a minimum of cash outlay.
Try looking at yard sales, thrift stores, and marketplace for framed paintings those size. Sometimes you can purchase for less than you can make them.
Here's a good tutorial on how to make a picture frame from crown molding: https://www.instructables.com/id/Making-a-Frame-from-Crown-Molding/
I've seen stretched canvases with just latice strips nailed to the sides.