Thanks for the reply, really appreciated.
Yep, I think the receiver below the speaker would look the best.
I am aware that my receiver can get pretty warm (onkyo nr515), so I have made sure with my dimensions that there is enough room around it, but I am still concerned.
I did consider that if the back panel didn't work, I could cut a bigger hole out and attach a usb powered fan (or two!).
I am now wondering if there is an alternative product / material that I could use for the back, i.e metal mesh???