Do you really want or need a seat?
If your thinking it will make for a comfier ride, I would say not.
I used to have a Honwave 3.8 airdeck.
I always felt safer being lower down inside the boat, rather than sitting
on the bench seat that comes with the boat.
What I did do was chuck a couple of these
into the boat to lift me up that little bit higher for better visibility
and to get my bum out of the water that inevitably slops around.
If your using the boat inland on a lake or something with no swell,
then sitting on the bench would be fine, but for being in a swell or some chop my advice would be to get a s low down in the boat as you can.
Hope this helps.