Well, looking at a few examples from my "Soviet Placard" collection below, here's what I would do --
Paint some squares of .005 styrene sheet metallic blue. (or aluminum, or black) Add model railroad or aircraft stenciling over it. Don't worry about what it says, or what font it's in. Just having the text in the right general shape will be good enough if you're looking at it at an angle throught the hatch. Anyone saying "Hey, that's not what it's supposed to say" really ought to get a job with the Secret Service running down counterfeiters...
Real Placards:


As for the actual stenciling, I find railroad decals for locomotives and rolling stock to provide almost exactly what I need in any given situation. You can make out white and yellow stenciling on this particular sheet. It even has solid black you can cut out and use underneath any of the stenciling.



A few sheets from different railroads should last you at least a couple of days...