I bought a factory seat cowl for my Ninja 400. I...don't think I'd recommend it.
Link to original page on YouTube.
I had a seat cowl for my Ninja 250, I like how it improved the look, and it was super easy to swap back and forth with the passenger seat when needed. I pretty much never put a passenger on the bike, but it was useful to have a flat surface for strapping on camping gear, like a tent...or a small stack of firewood. Both happened.
Unfortunately, the seat cowl for the Ninja 400 is a bigger pain in the ass. To fit it, you have to install a small bracket under the seat. And once that bracket is installed, the rear seat no longer fits. So if you want to go back and forth, you need to uninstall/re-install the bracket. Gross.
It'd be annoying enough if it was a simple tool job, but the bracket is not easy to install. I think the instructions want you to remove the rear plastics in order to get at the small space where the nuts + bolts fit. I started to oblige, but all of the bike's plastics are intertwined -- I got to the point where I was unbolting the front fairing just to remove the rear plastics and gave up.
The challenges: The space where the bolt head fits doesn't leave room much room for a socket, and the space where the nut fits doesn't leave room for fingers to actually hold the nut.
I was able to pull back the lip of the plastic and make enough room to jam in a small socket head. (Be sure to release two bolts from the rear plastic, which will give enough play to allow the socket in.)
To manage the nut, I stuck duct tape to the tip of my index finger and held the nut in place well enough to get the bolt threaded through. At that point, I could fit a small 8mm wrench into the small space to hold the nut in place, and use the socket wrench to tighten it down.
The seat cowl looks pretty nice, the butt bumper is nicely fitted, and I think it's overall a better-built product than the 250 cowl. But way harder to install, and much more inconvenient to own.
Thought I'd share.