This is a standard process for making seats in race cars. You purchase or build a simple seat shell ( like the Kirkey seats without the covers on them ) and sit in it while someone pours two part expanding foam into a trash bag behind you. When it's done the foam has solidified and you pull the foam out of the trash bags, shave it, and then put fabric over it.
It's really fun and not terribly expensive. You can also repeat the process as many times as you want until you get the cushions that feel right. After the foam has hardened it can still be supplemented with hand cut self adhesive memory foam sections if you find you need a little more support in a specific place.
During the few minutes that the foam is setting you would need to maintain good posture. If you can't do that you can find someone approximately your height, weight, and proportions to do it. This is where you can avoid making a "bad imprint".
I don't know how you can accomplish this with a seat that has a hinge in the middle so I'm afraid I'm no help there but you could leave the reclining passengers seat in the car in order to get rear seat access even with the fixed back drivers seat.