I think it would be inexpensive to remove the top assembly and take it to a boat/upholstery shop and ask them to sew in a new one. It is surprising easy to take off the top. I took off the top with the help of another forum member.
Oh man! Someone cut the plastic on my top ! - 986 Boxster Top Issues and Solutions - RennTech.org Forums
I wouldn't try to sew it in from inside, unless you are a masochist.