Why not make one (or a dozen) out of paper.. It's a box with a curved top right. Should be able to wip something up for that. If you have pics of the one you want. You could add paint (USMAIL) or even give it a bit of age in your favorite graphics program and print it out as many as you like. I'm thinking a plastic one would be less detailed and way more money..
I'll help you out on it. I may need some too?
I'll need Location (Country/Canada,USA) and Era your modeling as they have changed the boxes over time.
Your looking for something like this I would imagine.
http://farm1.static.flickr.com/195/441286244_32f8b44fc4.jpg
(Formulating a plan - shhhhhhh evil scientist at work mahahahaha)
From that image I can rough out the dimensions as it looks like it's sitting in front of building made with concrete block.. (CB's are 16" x 8").. Extrapolating and given the Governments need to keep things easy to build. Looks like...
Depth/Width 24" x 24"
Legs 10"
Height to Arch 37" angling to door at 38"
Arch Radius is 12" (1 half Box Depth)
Making overall Height 49"
Okay so Box is 2' x 2' x 4'-1" (1:1 Scale)
In HO that would be roughly 1/4" x 1/4" x 9/16" <= That's a pretty small box (HO)
Second Idea: (Scroll Saw + Doweling)
You could take a piece of 1/4" square doweling + 1/4" round doweling or 1/4" half round molding.
Box portion INCLUDING Legs.
- 1/4" Square doweling.
- Cut "Box" portion 13/32" (scale 37"). Cut a notch 1/8" deep leaving a 1/32" square on each corner (the legs). Doweling is hardwood so it should be able to hold the cut.. (not splinter very easily). I would start in the center and move to either side tilll you get to end of leg recess.
Arch and Door Opening.
- (1/4" Round doweling or 1/4" Half Round molding)
- Cut a notch 3/16" wide and 1/8" deep (that's for letter opening drawer). Then if 1/4 doweling cut in half length wise. Then leaving a 32" on either side cut arch at 1/4" long.
Glue both pieces together and paint...
Third Idea... (and so it goes 
)
A combination of both seems like a good plan. Doweling as a frame and paper for the details. Paper would be better then card stock too (so small it would make it easier to cut,fold and curve the arch portion).
Box (1/4" Doweling)
- Cut 5/16" or 27 scale inches long.
- After printing paper exterior (to make sure colors match the printed page)
- Paint white on the sides and color of box on top and bottom.
- (as wood color could show through printed paper surface)
This will give box a top and bottom and opening slot area will be colored to match. This will also mark the box so that you don't wrap the wrong sides with the paper. It will also give the box a bit of weight and make it easier to glue the rest of the box (the paper exterior) to it.
All that's left is art and cutting pattern for the box itself. For that I'll need the other info. Location and Year your modeling. The box itself is just a hair wider then 1/4" so when you rap it in paper should be bang on scale. Printing will give it that extra sparkle as it will have USMail or Canada Post or what ever logo was being used at the time. I can make it look embossed/raised or simply decaled on the outside. I don't think I worked so hard on something so small before LOL. Could be fun though..
It will probably take you a lot less time to make then it took me to write this post..
Let me know the where and when your modeling and I'll see what I can come up with okay..
