Here is a mod that I just completed to install some Mud Dauber screens on the Water Heater and Furnace panels. Just buy some Aluminum screen used on patio doors for about $7 and cut to fit your openings. Along with two replacement pop rivets (I used 3/8") and a little silicone/RTV is all you need to do this mod. Its a lot cheaper and more appealing IMO. For the Fridge, I just ordered the screens off Amazon since I figured the fabrication would be a little more involved and not worth the effort. I'll get them in the mail today and probably will be pissed I didn't just make them too. :-) Anyways, hope this helps someone.
Furnace after drilling out rivets and removing screws:
Cut screen to size:
Furnace Final product with two new rivets:
For the Water Heater, remove factory screen and sandwich screen under the factor screen and cut to size. For the intake louvers, just cut to size and I just used silicone to hold it in place. If it doesn't withstand the temps, I'll just use automotive RTV. This keeps it a nice clean look.
Final product of the Water Heater: