Tony, You will figure it out. It seems obvious you need some sort of center speaker turned on it's side to sit in that space. I always recommend going with the same brand speakers all the way around the room. If this is not possible at least try to match the brand on your mains (L/R) Every speaker manufacturer usually makes a couple different sizes for there center channel.
Once purchased see if you can add a shelf of some sort over the refrig, as not to pick up unwanted vibration from direct placement atop your ice-box. If you have to go to another brand speaker what ever you do DON'T BUY *BOSE* Good luck:~)