I suppose that what confused me was the multiple KO's on the fixture, arranged to match various boxes. I saw this as implying I was supposed to attach the fixture to the box.
Now, mounting this fixture on a flat wall, and leaving that large gap between the back plate and the box judt doesn't seem right. I'm then using the back plate as a spring to hold the fixture in place. With the 'keyhole' solts, it's too easy to then knock the fixture aside. I have this bias in favor of wanting to attach things tight against the box.
Had the fixture come with a little bracket I probably would not have been confused. Had the mounting holes not been so clearly arranged to fit a box I would not have been confused.
Had the mounting holes been located around the cord exit, installation would have been a delight.
I can't re-route the wires because the bulb gets in the way.
I've made other arrangements for the porch; what am I to do with the three lights I bought?
Well, I see no way to 'properly' instal them. I'll use them as lights inside my storage shed, where I can run my pipe and boxes around the outside, and feed just the individual wires out the back of the boxand through a hole in the wall, making my connections in the space behind the back of the fixture. If I do it right, the mounting screws won't be anywhere near the boxes at all. (The shed lighting is powered by an extension cord, only as needed).