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I understand the back support idea but I think that a back rest is just compensation for weak back and core muscles. The better conditioned your core muscles the less you will feel the need for a back support

I agree that having a strong core is a good thing.
For some of us who like the Double Paddle, a backrest is not just for support, it helps generate power when using it. Using a pack or foot brace to lock yourself in to position makes all the difference using the double.

I don't kayak, but from the seats I've seen the buckets serve that purpose in them.

I've used the tractor seats, don't care for them because of the issues KF/JJS mentioned, but if I wanted/had one, I think I would consider drilling some small holes (cautiously) for ventilation and too aid in drainage.