ISO a cushion to negate lower back pain

hey first time posting here. I have an emotion spitfire kayak and have had the darndest time finding a cushion, or alternative, to help my posture while kayaking. Ive tried a kayak-specific cushion that wasn’t nearly thick enough, a few towels that got wet of course, I can’t seem to find anything googling. I thought about getting a paddle board so I’d have the option to stand up and stretch but I’d rather not. I just need something sturdy that’ll raise me up so my pelvis is above my knees so my compressed lower back doesn’t get worse. please help thank you!