Thanks to Xerothermic for spurring me to give an update. The seat is not complete yet but close. When its done I will post some pictures.
Back in June I started modifying my seat. I purchased a stock spare while I am working on this one.
Cut out the foam and made a temporary cover with some "clearance special" outdoor fabric. I took the cover on and off multiple times to get the seat dialed in. I have new material to make a cover. It is extremely functional and cooler than the stock vinyl. I have done a 1000 mile day and only stopped because I was back home, my rear was not sore or uncomfortable at all. I have plans for 2500 miles in two days coming up and have NO worries about seat comfort. Here are a few pics of the first part.
I am not totally done with it yet but it is 90% complete. I ended up taking all of the foam out of the center of the seat. I left the edges so that I had the basic structure still there. I then I did one layer of firm and 2 layers of medium Sunmate foam. On top of that I put a layer of pudgee. https://www.sunmatecushions.com/
(look at the clearance items, its how I bought mine)
It took a lot of cutting trimming and shaping and re trying and more trimming, shaping etc. But I am very happy with the comfort of the seat overall. I still have to make an actual cover. At the moment I have a piece of the fabric I am going to use stapled on but it is not "fitted" yet. I am using a 3d spacer mesh. It is a very durable mesh fabric. Under that I have a layer of waterproof mattress cover fabric. It is a stretchy waterproof fabric (think baby crib mattress cover). This layer may change to vinyl depending on how it looks durability wise when I make the final cover. By that time I will have been using it for a few months and over 5000 miles. I will also be using a 1/4 foam for smoothing between the seat foam and the cover. Overall it is a medium difficulty project. Cost wise I probably have $150-175 in it right now and a LOT of hours.
Suggestions: When cutting the foam make it a lot bigger then you think and trim it to fit once it is in place. I had to do a bunch of infill because I cut it to small.
Personally I would go this route again, although it obviously is not for everyone to tackle. I have not done the passenger seat yet but that should be a lot easier. I ended up buying a second seat and used that to cut up etc. I am keeping my original seat just in case I need it in the future.
If you are someone who likes to figure out puzzles and make things this is a great and very doable project, if not finding a used ultimate, mustang etc would put you in the same ball park cost wise.