What can fit underneath the seat?
You can actually fit a lot of stuff under the seat in your Ninja, if you're organized and patient. You know if you're the kind of person who should put it all in a tank or tail bag instead.
Here's what one of our members keeps in his storage area on an ongoing basis. Note the Ziplock bags; water can get into that space.
Should you do any touring, it is recommended that you NOT keep your paperwork under the seat. If you happen to get pulled over, you'll have to move a lot of stuff to get at it.
This is one way to organize it.
The flashlight, wire cutter, and tire gauge go on top of the battery. The other 3 ziplocks (use multiple layers, or buy sturdier bags) are in the rear by the tail light, the maps under the left side cover, and the bungee under the right side cover. Extra fuses can go in the side areas of the rear fairing.
It has been suggested that you may want to find something to plug the hole that the license plate light runs out of.
Also, there's no need to carry the owner's manual. There's nothing important in there, anyway. All the specs in it are on the stickers on the inside of the side covers and the rear inner fender.
If you want to do some rearranging and leave some of this stuff out, it is possible to fit one of the portable air compressors under the seat. Break out your tape measure first.