Heater Blank-Out Panel

[Butsky]

Thinking on removing the old gas heater in our Boler 1300. The old outside grill vent is a bit beat up. Where can I find another(9.5x9.5 inches square) to put a sealing inner panel in between it and body to be waterproof? Want it to look, but not be, functional. A plain panel would look too obvious that something is gone? Is this size grill easy to find?

[Butsky]

Update: heater is fully functional but I never use it. Any market value or throw in scrap heap?

[Jon in AZ]

If it's a gravity-type furnace I think there would be a market for it. Boondockers value them because they are vented but require no battery power.