It probably depends a bit on what hurts her back, but in general sib seats (basically a bench) don’t encourage good posture, aren’t comfortable and provide no back support.
The tubes are the obvious place to sit with some cushioning but encourage a twisting of your back. Before suspension seats (which are clearly ridiculous for a T35) people generally preferred jockey seats for protecting backs. The idea being you use your legs to cushion the ride. To get the maximum benefit for that you need a solid handhold at roughly chest height in front of you.
In terms of DIY, improvised solutions potentially providing similar benefits in a sib I wonder if kneeling (on some thick foam for comfort and padding) astride some sort of inflatable sausage in the sort of position used in a Canadian canoe would help.? Not sure if the lifelines would provide useful handholds.?
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