For my 12’ x 30 home design and built canoe I use a SOT type strap in seat. Comfortable enough but could be better.
Two years ago a added customised bottom cushion and lumber support.
I wanted holes to relieve the pressure on the ischial tuberosities and coccyx. I say on a chair on foil to get the exact position right for me, the picture tell the story after that.
The actual pad is made from garden centre kneeling pads with a layer of camping mat on the top.
Them I used web data to make a lumbar support, with multiple layers of camping mat to get a 3D shape. It then simply Velcros onto the proprietary seat and works well.