I did something like that on my third day of Casita
ownership because I had to wash the thing down inside (see related posts elsewhere about that) and I really, really wanted to evacuate the fumes and hot air as fast as possible (it was early July and about 90 F in the driveway).
I hot-melt glued a bunch of old computer fans to a piece of cardboard with round cutouts for airflow. The cardboard was sized to fit inside the plastic-framed screen on the stock roof vent. A little careful placement of the fans allowed everything to clear the bar that runs across the middle and holds the gearbox for raising the lid.
It actually worked pretty well when I hooked it up to 12v. Certainly well enough to evacuate the thing over the next week during the cleanup process.
It has since been replaced by a MaxxFan deluxe (smoke, manual) but did the job adequately during the time I needed it.