You're really over-rating the ability of pistol rounds to punch steel! Consider also that 1/8" A36 steel plate weighs 5 lb./sq.ft. Hitting it on an angle, 1/4" plate would be plenty, but that's 10 lb./sq.ft. At a range I used to shoot at, they used 1/4" plate, angled to deflect the bullets into water about a foot deep.
Steel has also gotten crazy expensive.
You gotta use something though. I would start with a 4" x 10" sheet of 1/4" steel, and bend the sides so as to form the trap shape. You would have to talk to a steel fab shop for this, but they would probably take it on just for fun. It might give the shop owner some ideas as well. There are fab shops in every town. On your first visit, ask for a piece of scrap steel in 1/4", 3/8" and 1/2" so you can prop them up at different angles and see what kind of damage (if any) your ammo does to them.
I suggest that you make up scale models with cardboard, just to work out your ideas of how to shape everything.
