[vectorit]

Seat foam can be tricky.
You don't want to use just any foam, since there are different types that react to compression differently (your butt).

It would be terrible if you spent hours on getting a seat together with craft shop materials, only to find out that you sink into the foam so much that it is uncomfortable to drive.

I personally have beat this issue to death, since I have 3 bench seats to do and the cost is high.

Fortunately for me, all of my original foam is still pliable and hasn't had and rot issues on the back two. However my front bench, particularly the drivers seat for obvious reasons is shot. So, I am saving up for the proper foam work for just the front bench.
Just about any upholstery shop focusing on car/truck seats can craft a comfortable foam base, then they use layers of different foam and matting to get the form and comfort one would want in a car/truck seat.

Lot's if not all of the vendors sell pre made bench covers, that are budget minded and not all about OEM authenticity. Trouble is, that a "good" upholstery shop can make one to. But, what are they charging you per hour to make said cover? I have found that it's cheaper to buy a pre made cover, at least with my local shops and their rates. Your area may be different, so don't just go by my word on this. Do some shopping, crunch the numbers and see what pans out.

In my case, I am wanting OEM 67/68 vinyl/cloth "Custom" seat covers just because I really like what my burb originally came with. Unfortunately for me none of the vendors are selling them, and if they are they are not selling the rear bench seat covers for the Suburban. Let alone my unpopular color combo.

Do some research, and shop all of the upholstery shops in your area. I also ask people at car/truck shows who did their upholstery work, that way you can see the work first hand and get a customers POV on how the business transaction went.