Online Community of Zenith Builders and Flyers
If you type in boat seats in Ebay, there are about 4000 of them in different styles, heights, colors, etc. Very easy to install. You can use a thick L angle bolted/screwed to the left and right, or the front and back, and mount it to the seatpan or use the mounting holes that they have in the seat.
Seats are generally $150-175 for two, rather than the very pricey Zenith seats. They way about the same as a Zenith seat with cushioning, cover, and frame.
I got the idea from here. I did not modify the base as it was not necessary.