Since you have a coupe, a second set of hands can make extracting the drivers seat easier. A blanket / large towel over the sill will help to prevent paint scratches.
jwmelvin's method works. The other option is to source some foam in 1" layers (even better if you can find some 0.5" for fine adjustment). With the seat installed in the car (minus the bottom cushion), you can add and remove layers to adjust the height and experiment with the front / back cushion thickness. When you find something that you think you like, you can contact cement the pieces together for a longer term trial. At the point you are happy, you take your foam template and have it shaped and covered or have a new single piece foam hot cut to match and have that covered.
The other variable will be the density and compressibility of the foam. You probably want to start off with something that matches OEM. If you don't live in an area with some good auto specific upholstery shops with foam samples that you can check out finding the appropriate foam may present the biggest challenge to this experiment. In my limited experience, the foams used by residential / commercial upholstery guys typically have a lower density and compressibility than automotive foams.