Here's what I made for a friend of mine. I had a piece of glass on hand that
was 10"x10", so I made the box frame so that the interior space was
10.25"x10.25" inches, just big enough for the glass with a little extra for
easy fit. The sides of the box were 2.5" high. For the front of the box, I
used simple picture frame wood nailed in place with brads to frame the glass
in place and then used glazing points under the glass to hold it in place.
The back of the board was simple hard board. You might want to use something
else, maybe 1/4" or thicker so that you can tack any medals or ribbons to
it.

You said that you don't want to wing it, but that is mostly what you'd be
doing since not everyone has the same war memorabilia. Anyway you make it,
as long as the glass is clean and the frame is neat, it's going to look good
and will be appreciated.