You could build one. All you need is a vent over the pilot/burner area. We rebuilt a door over our generator. Shouldn't be too much different. We simply took the after market access panel that was on the camper, removed the old, cut up damaged piece of plywood from the panel frame. Bought 2 AC vent covers from Lowes (for a water heater you would only need 1) with fixed small louvers. Then, using a piece of exteror grade plywood,we cut out a square hole (
) that corresponded with the louver openings in the vent covers. Bought some aluminum flashing (in the roofing dept at Lowes) skinned over the exterior side of the plywood (for water heater, I would skin both sides with flashing... glue down real well with construction adhesive) and screwed the vents over the openings. Reinstalled the plywood panel into the panel frame. You should be able to pick up small access panels at most any RV store. But then you should also be able to buy a new panel door for your heater. It's probably compatable with an Atwood water heater.