From what I can tell the seat cushion is TPU material.
This method only works if the material is TPU. If not, it won’t work, and you’ll have made an expensive error. Do your homework to find out what the material is, surely the manufacture will have more info for you.
If it is in fact TPU, just heat weld a zipper with the leather iron, same method as found here.
Another option is using the epoxy “Stabond” the stuff smells horrible, and is difficult to work with, but it will seal a zipper to your para-seat for sure.