I have straps on my cars and have had no problems. I use the kind with the lambs wool backing between the leather and the paint, but no protective film on the car itself. I had one on my 71 Plus 8 for 10 years with no marking. I too like the double buckle but don't know of a specific vendor. Might suggest you go to tack shop in horse country and have one made. Might be less expensive as well.

Cheer,

Attached Images
71 Plus 8 w Belt.jpg (84.07 KB, 417 downloads)
This is right before I sold the car. The belt had been there for 10 years.

Mark

1934 SS MX4
1938 Series 1 DHC
1951 Plus 4
2005 Roadster