After some shopping around I have decided that wicker is to expensive. Another idea I had was going to the local saddler to have a look and see if canvas was an option. But the same here, basically the time spend would make it to expensive.
So this is what I am going to do:
I am currently creating a base for the seat using 2.5mm polyester rope, and weave it “inside” my square seat. On top of that I use some seat stuffing material, and cover the seat with leader. The side bits, leather as well, will be sown to the seat, and go up to the top rail and attached there. To make it affordable, I/we will be doing all the work ourselves 🙂