This surprised me. Assuming 1/2" plywood, here is my analysis of a 2" radius on your corners, It's amazing to me that it doesn't get more into the 1x3 support. You could add another 1x3 or 1x2 for more strength, but depending on how you attached it, you may just be able to plane (assuming no nails and screws that can be removed) or belt sand this without adding anything.
