Wouldn't that work for you? It might not insulate as well as a steel door, but it could be painted to match the wall. I have several access doors like this (a little smaller; maybe 3 feet) in my house (18th century) - I think they were put there whenever they had a pipe leak - after they broke open the wall to get to the leaking pipe, they framed out a nice teeny door so they'd never have to bust open the wall again there. It looks fine; can be a conversation piece too.