Excellent fit in the end but very difficult to install with the tool provided. The tool sent was defective, too hard to use to connect the spring. I ended up just placing the screen over the exhaust and closing the door and it works well. I think the connection spring attachment and tool need a re-design and better quality control but I highly recommend these screens for hole protection. They just need patience and creativity. Modify the tool as needed before application.Update 6-21-13:I used one of these on my Tankless Water Heater exhaust Vent as it had a large space around it and there was plenty of room for nesting in the exhaust and in the compartment. Since I need to access the heater switch to turn it on and off, the screen cover would come off the exhaust port. The exhaust pipe itself has a larger-holed screen within. I ended up screwing the supplied spring to the approximate center of the screen cover to shorten the spring and improved retention then hooking the spring to the built in internal screen (with a just-found previously provided screen-spring tool). The screen stays on now when I open the decorative unit side cover.The manufacturer legally has to tell you not to cover vents (to keep out wasps), but, as long as you periodically check and clean the screen it in no way restricts air flow. The decorative side cover keeps the screen centered in the exhaust hole, to screen it off from invaders and the spring retains the exhaust screen cover when the side panel is opened.There is also louvered intake venting. I ordered a large flat SS screen and cut it to size over the internal louvers with metal snips. There is plenty of air space but no place for wasps or mud daubbers. This solved my problem with the Tankless Water Heater unit. I bought the preformed screen for my RV air heater and it fit perfectly. If you are concerned about restricting air flow, just inspect it when you go through your set-up list and use a can of compressed air (Costco) to spray it clear of dust as you like. If you do not have a wasp problem don't use them. Hope this helps someone.