[QUOTE=OpalCat]
Well the main thing is it has to be wedge-shaped. If it’s just padding the back of the tub but maintaining the verticality of the end of the tub, it doesn’t help much. I need to be able to be in a more supported+reclined position, not folded in half at 90 degrees like my tub currently does 
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There are also foam bed wedges. And I believe you can also find them with waterproof vinyl slip covers. Although they are touted as waterproof, I’m not sure if that equates with submersible.