This is the way most of my structures are built.I start with foam board and draw on windows,doors etc.,then holding an Xacto # 11 blade at right angles I cut them out. (Save the cutouts in case you change your mind and want to re-glue them in.)
I then begin framing the wall with the straightest pieces of wood that I can find ,following the same methodology as if you where framing a 1:1 scale structure.
If you want a barnsiding type structure do not seal the wood and watch for excess glue stains.
In most cases for shadow boxes you will only be sheathing the interior walls.
Warning! when using foam board and unsealed wood ,warping can be a problem.Find a flat surface and weight the structure down flat with books,bricks etc.. overnight,when using any type of water based glue.