OK, I cut the huge piece (4' X 8') of Styrofoam to 47" X 47". I WAS going to cut it 44 X 44 and wrap the fabric to the back but then I thought of this:
Why not cut it 47" square - just one inch smaller than the piece of fabric and just do a 1/2" wrap. THEN I can add 1.5" wide lath along the sides to #1. hide the cut edge of the fabric, #2 give the appearance of a frame and #3. Have that 1/4" reveal from the front visually STOP the eye from falling off the frame.
This is the Styrofoam cut to 47" X 47"
These are 5 very straight and flat ratty and filthy old lath strips 48" long
Blurry but you get the idea. Glue the strips onto the sides of the stretched and finished piece
This will be the reveal from the front (look at the dirt that fell off the grimy wood - yuck!