It's a troubling task to preserve the originality of the cover. I have a file with dozens of images of hideous water heater “installations” that proves its aesthetic difficulty.
I mimicked the original 1968
Bowen louvered theme. I'm not thrilled with the boring appearance of mine, but it's highly functional, and durable.
It's pretty easy to make an aluminum door. A $10 sheet of .040 5056 is easily worked. An aircraft fabrication shop punched my louvers. At the Detroit Autorama there was a guy punching louvers for $2 each. My home-made door stands proud of the skin by 9/16”. It covers an Atwood 6gal, electronic ignition, dual-fuel. It has a Stainless Steel screen lining to prevent insect intrusion. You don't need the pilot hole with electronic ignition.
It's fastened with four ¼-20 socket-head button screws to foil unauthorized interests. I only open my water heater door once a year for Winterization.
If I had ZERO fabrication skills, and no interest in learning, asking a HVAC shop to make a door would be my solution.