I actually had just modded one of these for my cousin, and I must say, I was pleasantly surprised by the performance. Instead of hoppering it, I had opted to re-barrel the breech with CPVC, and that turned out well. I might just need to get one of these for myself and give this a shot, though!
Wow, you are thick. Did you even bother reading the title of that thread?
Nerfhaven Modification Directory, Closed - see version 2.0
And then when you get to Version 2.0, that's closed too. And then in the Modifications section of the forum, there's a pinned thread by the name of Nerfhaven Modification Directory, Version 3.0 And guess who it's written by? That's right, Blue.
If you can cut a piece of wood or plastic to fit that recess you probably won't even have to bother with making screw holes and tapping to attach to the plunger tube.
Actually, from what I can tell, it needs to be attached to the plunger tube in order to be able to push the spring forward. The plunger tube pulls that forward, which also pulls the spring forward, and when it's at the front, the catch releases the plunger rod. Without that piece being screwed in place, the spring won't be compressed, and the plunger rod won't have any power.