No answers yet?
Might try the PRO Desk @ Home Depot
to get that exterior door + installtion
THEN get proper size gas vents for the installer
to use on the walls (easier than cutting a metal door).
SOP Tip:
If the water heater needs more air than is available in the room, you will need vents or a louvered door connected to another part of the house or the outdoors. In most cases, you will need two vents, one located within 12 inches of the ceiling and another within 12 inches of the floor.
Home Depot PROs can help figure all this out.
Since I'm blind to all the details, locations, nuances ... this is only a start, perhaps something to leap frog with ideas.
Ten words or less ... a goal unattainable