I am using an expandable RKA seat bag on my 2014 Zero S. What I like about it is that it uses a fabric "plate" that stays on the seat and allows the bag to be unfastened by disconnecting four Velcro straps. The quality is very good, but the price is a little steep, compared with Chinese-made seat luggage. However, there are so many seat, tail and rack bags on the market that I am sure you will be able to find luggage with the size, features and price that you are looking for. Attached is a photo of the RKA bag on my bike.
