Boaters have dealt with this problem for years, of having to close deck hatches that were left open for ventilation on stored boats. Here's what some of them use…
You would have to leave the fan cover's arms loose, so that it can fall shut under its own weight. Then you use this device to prop it open. The device uses an ordinary aspirin as a water-activated fusible link. When the aspirin gets wet, it dissolves, causing the device to pop out of place so that the cover closes.
Not sure it would work with a Fantastic Fan cover, but it's worth a try.