I cut out part of the dog (Kurt), flipped it, blurred it, and enlarged it so that the right side approximately matched the left side of the cover image -- a kind of butterfly effect.
In this case, opening out to the left.
I haven't talked about back cover, but they can be difficult also. Sure, sometimes a plain background for the blurbs and barcode is what you want. But not always.
I am fond -- someone might say "overly fond" -- of mirror-image front/back covers -- with the back, of course, indistinct.
The Staked Plains