The only thing I would have done different is stack them vertical and not horizontal.
I found there is a spot a specific distance from the ground that is the "sweet spot" this is just where it is natural to hang the target....usually it is in the middle....we don't want it too high because of the what if factor....or too low as...it just seems too low.
Now if your wood is actually stopping the pellets you are going to sooner or later chew that up and need to replace it.....here is the pain of it all, unscrew all the others to get to the ones that are chewed up.....where is if it was stacked on the vertical you can just take the few off that get messed up....or move them around easy....say the outsides to the inside before they get too bad.....I think I explained this well.
What I finally did was get the backhoe out and dig into the side of a little rise....then put something like you built on top of the hill for that "extra" protection, and shoot into the hill where I dug....now I am shooting into mother earth and the wood "backstop" is there just incase something wants to fly out. Now I generally shoot powder guns there....the airguns get shot into the field that has a nice rise....I have found that even my 25's loose so much after the first few hundred yards, I am not really worried about the next thing a mile behind the house.....with the lay of the land no way a pellet is going to get there.
All kinds of really cheap things you can do to extend the life of your backstop....and some are as easy as hanging carpet over the backstop.
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