Whitey,
I had a copy of the old NRA reprint book for range building, etc, and IIRC, it had small, portable style backstop (bullet trap) plans, and had plans for outdoor ranges and their earthen-type backstops, but not what you are asking about. I'll see if Dad still has his copy of it kicking around and do a check to see what's there.
I DO have access to all of the NRA mags back to 1926 or so, and if anyone can cite a month and year where they published such a plan, I'll be happy to look it up and scan it for board members who want it. Also, somewhere in the dark recesses of my computer I have a set of scans from a pre-WW II catalog for carnival-type shooting galleries. It includes several pages of neat targets that, while no longer available to buy from them, could be made today with a computer driven plasma cutter.
Froggie