Ventura rack J bike
A Ventura rack is a good way to make your commuting or touring experience more pleasant. There is one designed specifically for the Ninja250J. It's sold by Ventura USA. If you would like to see how the Ventura rack fits on the EX250F, click here.
This page on the Ventura site tells how the system is designed. You have to install the L-Brackets first. These are the steel rails which attach to your bike. Then, there are a variety of racks which fit into the L-Brackets. You can buy one or more of these. The bike shown here has the Sport-Rack.
There are two allen bolts that go through the frame under the rear passenger seat that connect to the bracket under the rear fender. As you can see from the pictures, the front part of the rack attaches to the rear passenger peg.
It is an extremely sturdy rack. The rack instructions say the capacity is 20 pounds. Unless you overload it, it shouldn't bend.
The Sport-Rack can be turned around so that it sits above the seat. In fact, this is what Ventura recommends you do when you're not riding 2-up.