@ashrauth,
I'm having this GIVI 46 ltrs topcase, which would cover your requirements. It's solid, removeable (plate stays on the carrier) has a handle....
Well, brings a lot of weight on your rear and looks a little bit too big for a small scoot.
The OEM Kymco (white) has around 30 ltrs and is just a jota too small sometimes and is fixed on the carrier. In practice any topcase available to you would work. There are some NoName out there, which fulfill all...all depends how much you want to spend. If you have the chance, go somewhere and check up on several types and brands.