Start by making or buying your new seat base and cut the 2" foam to fit. I cut one piece (10.5" square) and wrapped it with muslin to put down into the steel ventilation cage. I am hoping the muslin wrapping will keep it from desiccating and falling apart a while longer, and keep it cleaner.
Reconditioned seat base ventilation screen
New seat base bottom
Cut foam and cover
Cover foam with batting
Cut the 2" foam for the seat base by tracing the pattern from the seat base itself and then cut with an electric carving knife, band saw or serrated long kitchen knife. There is an angle where the two pieces meet at the junction of the riser at the nose of the seat bottom. I chose to make this in two pieces and pad over it, but you could use one piece and just force it down into place - which might be smarter in the long run. Trim the foam to shape using the base, the old seat cover or a picture of the finished seat, and your eyeball as a guide. The foam should be compressed about 30% or more when done, remember. If you wanted a firmer seat you could start with 3" foam and really squish it down when you put the cover on later.