I don't think that you will find anyone that makes a rack that accommodates a top case that is plug and play for the TW200. You will have to adapt something. There are several manufacturers of racks that work on the TW200. The sturdiest rack and most popular amongst the TW riders is made by CycleRack. After you purchase the rack then you can figure out how to attach a top case. I have a Top Box made by Happy Trails that I put on my TW.
Before I got the Top Box I used a milk crate - not very sexy but it was quite functional and cheap.
Happy Trails Top Box
I hope this helps.