Monolock base:
(http://www.scooterluggage.co.uk/images/k628.jpg)
The base rack of the Givi E181 is steel and mounts in the OEM tail rack holes. It's actually two parts, the K bike rack (steel) and the Givi base plate (plastic.) The nice thing about it is that the Givi base rack is that, since it's level, you can attach lots of different aftermarket cases to it or, in your case, some sort of bracket for supporting a backrest. (If you need a backrest then drop me a PM. I've got one.)
(http://www.motorcycleplanet.co.uk/images_products/givi/givi_E181.jpg)