I got an answer from you from my husband, LOL! He said make a template of your chair bottoms then trace that onto a piece of wood. Cut out the wood (hopefully it is straight cuts)and then you can add batting and cover just as if it was a seat pad that came with the chair. Then just screw the new seat onto the chair bottom. I hope you could understand my simple instructions because I assumed you know how to cover a chair bottom that already has a covered seat.
I got to thinking about the wood piece. You could modify your chair bottoms just a tiny bit to get the straight cuts you need to cut the wood pattern. We don't have any fancy saws or anything so I would probably modify ours a bit so we could cut it ourselves.

I did it with a lot of help from the Foam place. It's like an Upholstery and Foam Supply kind of place. I brought the chair seat to them and they cut high density foam to fit. It's about 2 or 3 inches deep. The old seats had an old leather insert that was in the center of the frame. He sold me some upholstery webbing and a tool to stretch it across the open area where the leather used to be, and I stapled it down. Then the foam sits on top and then staple the fabric down to the underside of the wood. I never would have figured this out, but the guy was really helpful in explaining how to do it. Cost $100 for foam for four chairs and webbing and tool. Worth every penny because those chairs don't kill my back any more.