I also have a 99 Windstar. Warning, there is 13 inches between the roof support structures in the back where your sketch shows the 14 inch maxx fan. The back door opens enough so that if the maxx fan is closed the door hits the back of the vent. I made a 2x2 lift across the back that tapers to no lift across the front. The only problem is that the fan control is hard to reach. The remote control version might fix that.
My first plan was to use a 4 inch computer fan mounted above the roof with a homemade tupperware hood. That might have been better. At least it would be possible to take a 4 inch hole up to 13 but the other way won't happen.
I mounted the one 40 x 26 panel to the roof racks that came with the van. I put 2 holes in each cross piece and tightened the nuts just enough to make it snug without crushing the cross pieces. Then I locked it up with jam nuts. Only 100 watts, no refrigerator, just my very efficient home made ice chest.
PM me if you have any questions.