I like your solution, and it looks pretty sturdy...
But check out what I did. I started making these back in 2012, and it's a "drop in" part that fits inside of the void of the buttplate. The only tool needed to install these is a screw driver, and that makes it really easy for the end user.
Every once in a while I'll run a production batch of them as a solution to the sling problem. https://www.youtube.com/watch?v=JYfXFbvzgxo