sew sew sew ! ive done a few where i just took some foam and built it up to help my boney ... out , then i took some spray adhesive sprayed the foam , layed the fabric on it and pulled to the otehr side of the pan and glued it some more and then if its a double pan seat clamped them together and its done .
another method is to sew one up they are so easy to make you can use your original seat to make up a pattern and go from there , kinda looks like you dont have a double pan seat coming to you so that is a little tuff to finish it off you can always tuck the cover under the seat pan and try to use gorilla glue ,
ive used t-shirts, shop rags,jackets, all for bicycle seats diy style , there are other tutorials up here i think about seats .
and if you dont have a sewing machine to use , and want to sew by hand i recomend picking up heavy string or .. do it like the punk kids do and use floss that stuff is amazing