The QW3 is a friction fit, universal Nerf stock designed in Tinkercad. After printing it requires some assembly and glue. For the front piece I modified and used the 'Nerf Retaliator Stock Extender, Mk.2' by bitsandbooks. The stock is currently untested but when my Rampage arrives, I will test it.
Update 1: (17.08.2019)
Oops. I forgot to add the file for the assembly plates. If you downloaded it before I added them, be sure to download them as well. They help the stock to be easier to assemble and sturdier after assembly.
Update 2: (17.08.2019)
Boom! Two updates in the same day. All of the files are enhanced and upgraded for better and easier assembly.
If you want, you can print the assembly plates at 100% infill. I used PLA.
There is two back options [Back (Ragged Plate) and Back (Curved Plate)]. Only one of them is required for assembly. Choose whichever one you like. I printed the ragged plate one but I think curved plate is more comfortable ( although I didn't test it yet).
After printing you need to assemble the pieces. I used hot glue but I recommend using something stronger. Glue the pieces as shown in the picture below. When gluing, be sure to align the pieces properly and centerly. And then if you want, you can paint your stock. I used blue and orange spray paint to paint mine. Just be careful using spray paint.
Gluing Guide (White Lines Mark Where You Should Put Glue On)