My husband and friend are in the process of mounting a 48 liter top case on a Honda Silverwing.
It is a Coo Case from TwistedThrottle.com Here is the web site
http://www.twistedthrottle.com/article/view/371#KymcoAs I read the chart, there are several sizes for Kymco.
To date, they are happy with the design and construction of the CooCase.
Mounting is via a plastic plate that comes with the case. It appears to be sturdy enough and has many "holes" for the mounting bolts.
There is a lack of instructions, but TwistedThrottle.com has made some videos that help.
Their next part is to do the of wiring the lighting system and the remote/automatic lock.
This brand of top case may be of interest to other scooter owners.
(I have been following the Downtown 300 discussion with interest and thought I should mention this brand)
Scoot Safely