Here is my low-cost dust cover for the water heater. Mine is on the back wall so it gets caked in dust which I believe was the death of the original thermostat/controller unit in the water heater that I just replaced. I got a car door magnet like you use for a business ad on the side of your vehicle and cut a piece to go over each vent. For the left side vent louvers the magnet is on the inside, for the big screened vent the magnet goes on the outside. I've been driving around for weeks with them and they stay on just fine. I just need to remember to take them off when I fire up the water heater. You can find this stuff on line for several dollars per sheet.
Cool idea, Central Toast.