Thinking of gutter mounts and such. If you cut a piece of 3/16" or so metal to the shape of the gutter in an area you can spread the load out allot. I'd try to run in 6" to 9" along the gutter. May take a bit of grinding to get it to fit nicely. After you have it shaped properly, take a piece of rubber to go between the gutter and the piece you shaped. Use hold down clamps like the ones in the
link.
As a side note, for cars that didn't have gutters I've shaped the sides of 2x6s to the contour of the roof right next to the roof edge then used foam rubber padding from backpacking insulation pads between the wood and roof. Hold down was via clamps into the top of the door openings. For cross boards I used oak flooring that I had left over. I put a piece every 6" or so. It could carry 300 lbs or so without any issues.