Here's something to think about. Since you can not seem to tune the antenna to an acceptable swr,have you considered using an antenna tuner to provide a better match between the radio and antenna?
This has been done for years in a variety of mobile installations where there are fiberglass components not allowing a solid mount.
Rather than destroying the p.a.in your radio,try providing it with a "tuned" load? Someone also suggested a marine antenna that doesn't require a ground plane.
Here is a source for some ideas. Good luck!
www.mfjenterprises.com or
www.ldgelectronics.com
Both of these sites offer some quality hf tuners.
N9ZAS.