You can get inflatable seats that install into a Zodiac and fixings for a hard deck. HP decks have them on already. They're right angle seats and are muchos comfy, cost about 90quid and keep the weight of the load nicely centred.
As for speed tubes, they do as they indicate and a don't offer a smoother ride in chop as you have to slow down anyway. If you blast it through chop with speed tubes you tend to start flying!
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