[MKS427]

I added lumbar support to mine. You'll need 2-office chair lumber cushions (that you like, test a few in a chair), an electric carving knife, Spray 77, 2" blue tape, and some patience. This worked well for me.

Cut the cushion to fit the seat back for width. Taper to fit to side bolsters the best you can.

Open bottom of the seat back.

Slip support in and tape in place.
Place seat in car and test.
Remove seat and adjust cushion height.
Repeat until you like the feel.

Tape the seat cushion around the lumber cushion generously to create an alignment aid and protect the seat from spray glue.

Spray glue per instructions.

Reassemble.

You could do something similar to the bottom seat cushion. I suspect that the lumber support will push weight off of your butt, especially if you are taller. You can test with a small towel rolled up, before you start making changes.