Brian, I know what you mean about not wanting to do anything to your nicely painted surfaces, but I think to withstand any kind of wind buffeting you will need to find some secure way to fasten them in place.

Two thoughts:

1) Years ago I had a bugeye Sprite with sidecurtains, and they fastened in with knurled knobs that went horizontally into the top/inside portion of the door. The actual sidecurtain was a sliding plexiglass deal in an aluminum frame with rubber all around the perimeter to seal it from the elements. The bottom of this framework had two aluminum brackets that went down inside the door, so that when you were inside the car they were exposed and right next to you. You loosened up the knurled knobs, slid the brackets home (they had slots in them) and tightened them up. This car was my daily driver and I lived in Pa, and drove it in many snowstorms. Point is, it kept the inside warm.

2) You could put some nutserts into the top of your door (I know, it's all painted pretty and all ) but they would be pretty unintrusive because they are sort of a gold color, like grade 8 bolts. You could put one in the front of the door and one at the rear, and put a thin rubber washer like a fender washer down before you affixed some kind of bracketry to them to hold your fabbed up sidecurtains in place.

Of the two, I like the first option better as I think it would be stronger and less noticeable when not installed. You could put some foam tape on the underside of the framework where it would rest on your door top to protect it from chafing. Just some ideas.

However, this trip will be in July, and I think you guys get above freezing for at least THAT month.

Don