I wonder what this critical life threatening need for combustion air is about?
Simpson the Bvent manufacturer wants the dilution air to protect its vents from corrosion.
Saying
" Ventilation of any type of fired unit is not something to take lightly. After all it is you and your families lives at stake. Don't mess up."
is silly.
New code standards for a confined space in a energy star level home ,applied universally are ludicrous in some conditions.
The math for burning gas shows a need for half of what the minimum amount is by the 50 cubic feet per 1000 btu's rule.
Half is what you really should have. Leave a little room under the new door, "the undercut"
Use a normal looking register that matches the rest of the house if you want a hole thru the wall and move on . It's only a water heater.on for maybe 20 minutes twice a day.
When I think about a turkey in the oven for 6 hours and 4 burners cookin , wheres the ventilation air and venting then?