It seems that many replacement foams are thinner than original. I had the same issue with a set I did years ago. A little trick I learned years ago from an upholstery guy I knew when I was restoring a volkwagen beetle I had was to add an additional layer of foam under the new replacement foam to give it a little more cushion and build it up to keep the cover nice and tight and avoid wrinkles. He also told me to put the covers in the hot sun or in the dryer for a bit to heat them up (not too long obviously, just long enough to make them more pliable). This really helps pull them around the base and stretch them out to fit nice and tight. I was impressed at how well it worked on my car and made the normally rock hard uncomfortable bug seats actually pretty nice. Seats came out nice though. Excuse the non mopar picture.
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