Chrysler fitted the later PT's with a small "lip" spoiler on top of the hatch in later years. I bet they are lift- & drag-reducing.
So if I had one of these cars, I'd swap for the lip spoiler. (Well, if I had one of these, I'd do an A-B-A comparison of both types first
... no, then I'd probably abandon spoilers altogether for a kammback or boat tail)
Maybe you can find someone on a PT forum willing to do a swap.