In conjunction with a 14 inch roof vent fan I have installed a couple of 4x10 inch on each of the front door kick panels. These are just plastic register vents made for home heating systems that can be opened and closed.
They fit perfectly between the underbody metal braces on the side panel.
I just carefully measured and cut out the steel panel with my 4" angle grinder.
I then reintroduced the plastic kick panel cover and marked where to cut it out.
I installed two layers of filters to keep out debris and bugs.
I should now get good ventilation even with all the windows closed, pulling cool air from under the vehicle in the front and exiting in the rear of the van from the ceiling fan.
I think the ceiling fan will move around 900CFM and the high top extended E350 is about 450CF, so at half speed the entire inside air will be completely changed every minute.