I didn't like the look of the seat that came with the kit, so I bought a "mild" Hot Seat. I added a layer of camping-pad foam. Instead of gluing it down, I contact-cemented velcro strips to the boat and seat. If it works well, I'll leave it, or I may end up gluing it down permanently, once I decide on the seat location.
While the camping-pad foam was comfortable, it unfortunately absorbs water, so I have since replaced it with what I should have used in the first place: minicell foam. I carved/sanded indents for my tailbones, tried it out today, and it seems comfortable enough.
The Back brace was easy & straightforward to install.