So, as usual and first of all, sorry if this as been asked, if I’m asking this in the wrong place or if this question makes no sense…! 
I successfully added a particle collision behavior to a Niagara FX I’m making (Spiders walking on mesh distance field), but the tutorial I followed for those collisions has the neighborGrid3D attached to the emitter using the Engine/Owner/Position parameter and doing some matrix maths.
I’d love to be able to scale some cube/volume in my scene and make this my neighborGrid3D bounds, to be able to have my emitter where I want, but the “zone” where the collisions are triggered also where I need! And… I have no idea how to do it! 
As it would be just a visual representation of my bounds, I first though a volume could be better as it is just represented in wireframe in the editor, but I don’t even know if it’s possible, or which kind of “volume” I should use. So for now I was trying with a cube, which I referenced in my niagara system with a static mesh parameter, then I tried to access it’s meshBoundsWS, and then I’ve reached the limits of my knowledge and don’t know how to translate this into a matrix (which is what I need to feed the “matrixSimToUnit” parameter.
I understand it’s quite specific, and I understand I may be far away from the correct thinking here, so any help or direction would be really appreciated!
I hope at least I’ve successfully explained my problem!
Thanks to whoever took the time to read this!