Haha wow! I haven't seen a Y-frame since 2017. Cool project!
I don't really think there's a right or wrong placement for the reciever module, as long as it doesn't short out anything. You also wouldn't need it at an accessible place because it's mostly a "set it and forget it" component.
What I've seen many people do is mount the antennas parallel to the motor arms (so you could put them on the two front for example). However since you've bought (in my opinion a way overkill) dual band reciever with diversity, you would like to have both antennas in a perpendicular orientation to maximise the efficiency of the diversity module (for example one lying flat and one pointing up).
Here's how I would mount them, shown in a crappy paint drawing: