I am tall, but I use my dining room table for sewing so I too had to raise up my chair height. Part of my problem is if I sit too low, my knees are up in the air! Not very good ergonomically speaking. It makes it hard to get up, out of the chair.
I found some pillow forms at my fabric store that are about 15" square and 4" thick. They are quite firm. I put 2 together and made a zippered cover to hold them together. The fabric I used is upholstery grade.
Now I'm considering bringing my "pillow" to church too!