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These are the ones I bought that fit well in the low spot where the couch cushions meet. I didn’t buy them for that specific reason but It seemed to work. At least for me…
ComfiLife Orthopedic Knee Pillow... https://www.amazon.com/dp/B07MZSTC1J?ref=ppx_pop_mob_ap_share
Cooling Gel Lumbar Pillow for... https://www.amazon.com/dp/B07PQ9YFD7?ref=ppx_pop_mob_ap_share