Before I built my barn in 1989 I did a lot of reading about passive solar. From that I decided to include openable windows at the peak of the open beam ceiling. They are opened in the spring and closed in the fall. A open window up high in a building with a source of air down low will create a natural draft. Hot air rises. That worked so well when the house was built in 1998, it also included a cupola with openable windows. The house windows are up so high the opening and closing is electrically powered. This does an excellent job of cooling the house in the summer in our climate.
Using what was learned from the barn and house, I suspected that it might work in a van. In the Sprinter conversion a 3 1/2" x 4 1/2" hole was cut in the floor just in front of the left rear wheel. That location was chosen so air flows past the refrigerator coil on its way up and out the open Maxxair roof vent. Wanted to increase the efficiency of the refrigerator so it would run less and thus reduce the electrical load on the van. The hole in the floor works. You can put your hand down by the floor hole and feel the air flow. When air under the van in the shade is cooler than the van interior you will get air flow. Normally do not power the Maxxair fan. Get air flow without the fan running.
A 4" square hole was cut in the Transit floor and then a frame was fabricated around the hole above the floor. The frame is made out two 4" long pieces of 1 1/2" 80/20 and two 1 1/2" square wood blocks that are 7" long. The end center holes of the 80/20 are tapped. The wood blocks are bolted to the 80/20 end holes with flathead screws. That creates a 7" x 7" frame with a 4" x 4" center hole. The 80/20 has a no slot top surface and a slotted bottom surface. Two 11/32" mounting holes were drilled 3/4" from the floor hole edge centered on two sides. Two 5/16-18NC carriage bolts in the 80/20 slots bolt the frame to the floor through the mounting holes. Under floor is a screen made from a house perimeter vent screen ("Hardware Cloth"). Screen is required to keep out the animals. So sandwich has the frame on top, the 1" thick floor in the middle and the screen under the floor.
The wood frame blocks have two holes drilled for two 5/8" diameter wood dowel posts that are used as quides for the 3/4" thick plywood hole cover. The dowels are 8" long including the 1 1/2" in the wood blocks. The method of opening and closing the opening is a replacement gas lawnmower pull cord with a plastic handle. The cord goes through hole in a cabinet wall and then through a 1/2" conduit 90 degree bend down to the center of the hole cover. The rope goes through the cover hole and is clamped with a bolt and fender washer to the bottom of the cover surface. A aluminum angle with a notch is used to hold the handle in the open position. Pull on the rope and hook it to the angle to hold the vent cover in the open poition. Remove the handle from the angle and the weight of the cover closes the floor hole. Can be operated while in bed on bed plarform or from the aisle.
It was interesting that the vent does not work very well with the hole open and the front windows partially down. I had the hole in the floor before I installed the Maxxair roof vent. Works much better after the Maxxair roof fan was installed. The open hole and open roof vent will keep the interior very close to the outside air temperature. I know immediately when entering the van on a warm day if I forgot to open the hole and the Maxxair cover. If the interior is hot I did not open the vents. With the hole and a Maxxair the van can also be vented when it is raining. On cold nights I close the hole and the Maxxair. I seldom power the Maxxair fan.