You’ll get better results if you make your geometry fit better. Try using a number of segments in your circles that lets you cut the inserts on an an existing hidden edge. Also break the horizontals at top and bottom so the end sections retain their shape better.
Try using the Dave method.
Here’s my version of your bend using the Dave and adding in the horizontals cuts top and bottom.
Strain Relief - 90 degBox.skp (1.0 MB)