Do you have any sewing or tailoring experience? It's not an easy task to do yourself but it can be done. BeattheBush on youtube shows how he did his. He hand sew the entire thing. The results were not perfect either and it took a long time.
https://www.youtube.com/watch?v=M7aS9NmAErA&t=784s
You can usually purchase the vinyl plastic window (for boats) and have it pre-cut it to size. Call your local vinyl upholstery shop. eBay use to sell it for around $70 replacement vinyls but I don't know how good the quality is on that. I went to my local shop to see the types of vinyl plastics offered and had it professionally done and paid $700.
If I was to do it again, I would stalk for a 03-04 hood with glass and replace it myself. If you like the vinyl plastic instead then it would be cheaper. I recommend looking for 01-02 because in 2001 Porsche added the extra lining as standard to make it more quieter. Before that you needed 2000 Boxster S top for it. Either method I believe will cost you less than replacing yours with a brand new top. In some cases even, it will be cheaper if not similar to replacing just the vinyl plastic itself. Since you replaced it 4 years ago and your vinyl has already cracked, consider glass which helps during winter drives and no more the Boxster chops to prevent cracks and creases.