Insanity, that should work ok, as long as you also protect the face of that box. If I had to design one on the cheap, but look good, I would use white MDF, and use brackets and BOLT UP The corners, top, and bottom, as screws don’t work too well. Also some construction adhesive prior to attaching the corners, top, and bottom. Line the back with 1/4” to 3/8” rubber mat, and have a carpet curtain having in front of it about 4” away, to slow down the pellet and act as a stop for any bouncing off of the rubber mat. Home Depot has 1” wide, 1/8” or 3/16” flat bar steel, and I would attach that to the face of the box. Then now you can use magnets to hold your target in place. Look at my post on my pellet trap and you’ll see the magnets I use.
one of these days, I’d like to try archery targets. The ones like 2 or 3” thick, that stop crossbow bolts like right now, and seals itself around the shaft. I have one from days long gone, when I used to own a crossbow back in 1988. Kept the target, at its like 24” wide by 30” tall. Still haven’t gotten around to see if it’ll stop a pellet in it’s tracks. Maybe someone here has tried it? All I remember about that target, it was a pain to try and remove a shaft out of it, cause of its sealing properties.