The correct hard foam will never be found in any Auto Trim Supply place ~ go to a high end Upholstery Supply House and ask for black foam , you'll love it ~ I use it in all my Vehicle seats and everyone always comments on how comfy they are.....

Your local Thrift Store will have a cheap electric carving knife that'll shape it easily .

Or , just buy a new CT seat from The Good Dr. ATV ~ Jack has the correct original Honda seats for the right price .