This is a replacement for the undersided stock of the Nerf Sledgefire so that it can be used on a sling.
Disassembly of the Sledgefire is required to swap from the stock to this, but you will not need to cut the shell to make it fit.
I recommend printing this at high infil, maybe even 100%, to ensure this is solid.
I have included the .rsdoc for Design Spark Mechanical so you can build on this if you like to.