I installed the Corbin low seat (which was hanging on my dealer's wall at the time) and found it to be wider but just as hard as the stock seat for my tender butt. So I then installed a Butt Pad that I had in my closet. It worked OK on my BMW (another maker of hard seats) but it still didn't really help much when placed over the Corbin seat and it was really too wide for the seat, in any case.
For years I have been hearing motorcycle owners on the internet rave about AirHawk motorcycle seat pads, but the ones that I had seen were still too wide for the Zero's narrow seat. Then I saw their "Cruiser Pillion" seat pad advertized on Amazon for $77 and bought one. This is their smallest pad and the only one that they sell that is small enough to fit the Zero seat. Its dimensions are 11" long and 9" wide. It is just the right size for the Corbin seat and should also work on the stock seat, too. The pad consists of a series of air bladders, each about the size of a canned biscuit, that are inflated with your breath via a valve at the rear of the bladder. The bladder assembly is enclosed in a fabric case that is fastened to the seat with two thin adjustable straps. While I haven't gone any rides yet, just sitting on the pad seems quite comfortable. You can adjust the comfort and height of the pad by letting out small amounts of air. Attached is a photo of the pad installed on my bike.