I have worked with short statured folks for many years. I have used many different types and heights of seat cushions. I have put cushions together and created customized seat and back cushions for many a need. In most recent years I have used the Sacro-Ease cushion as the foundation and had the vendor add foam and plywood as needed to get just the right seat height we needed for the client. Today in walked another short statured client with spina bifida with a severe S curve. I told the father I thought I could teach him to drive but I didn't know how I was going to get him up high enough even with multiple cushions. The next week the father brought in a tractor seat to consider. It was a firm foam cushion with vinyl covering. To my surprise, once I tried it in my evaluation vehicle, I saw it gave a firm surface for him to sit on though I still had to put it on another firm cushion. Anyway, it works beautifully and gets him up to see about 4" over the steering wheel.
So, if you ever see an old farmer selling old tractor seats off the side of the road, you should stop and pick up a few. While you're at it, buy a few for me and I'll pay you back!