For those of you contemplating changing the foam in your seat, I have an idea that helped me.
When you remove the seat cover you will discover that the seat pan has a number of ridges and low spots that appear to add strength and rigidity to the plastic. I believe that these ridges in conjunction with the cheap foam go a long way to the discomfort that our seats are famous for.
When diamond jim rebuilt his seat he used expanding foam spray to fill these in areas. If I understand his post he did a lot of grinding to remove the excess foam and smooth out the seat pan. I know that I did! As the foam expands you have very little control over where it goes. After the third application of spray foam, trying to fill in the low spots, a light bulb went off. The wife had a Teflon ironing sheet about 15”square. I apply foam and wait for it to rise (about five minutes) and then cover it with the Teflon. You can now work the adhesive foam into the voids and low spots with your palms. When the foam sets ( at least an hour) up you simply peel off the Teflon sheet and reapply foam as needed. Way easier!
I hope some of you find this helpful.

JohnBoy

I would imagine one of those "cheese graters" that you get to form plastic body filler before it's completely hard would work pretty good too.