Trunk
Not sure how much you want to spend.
http://www.scootworks.com/shop.cgi/page=partstrunk.htm/SID=1283216296.26382
If you go with the Scootworks "Super Trunk 3" you will have from $500 to $1,000. I have been looking at Super trunk 3. Trunk, Passenger back rest/ arm rests would be $725. If I go with their mount, and not, put it on my luggage rack $996. And of course there is also the optional $123 top mounted chrome rack.
If you can improvise a little, you could mount your presant back rest pad on the trunk. And, if you also have a luggage rack you could mount it on the luggage rack. The cost then would be $480 pluss whatever hardware you bought at the local hardware store.
There are trunks on E Bay for about $200. They need to be mounted on a luggage rack. They have plastic hinges and one metal latch. The Super Trunk opens from the side and has metal hinges,and two latches.
At presant I just use a "Rally pack" on my luggage rack.
http://tsukayu.com/products.htm
Tsukaya also has trunks and bags. I like the square style better.
Dave