Quoted from pindude80:No-extra-balls, thanks for the reply! I agree it is pretty minimal but my concern was when I was moving the game I saw the back and bottom flexing apart and didn't want it to get any worse.
I had also thought about getting some wood glue in the gap but I'm not sure how to go about getting the glue in there because there isn't much of a gap on the inside of the cabinet; the gap is the widest on the bottom side of the cabinet and this would be the optimum place to put the wood glue but I don't think that would work because the glue would probably all run out. Maybe I'll try to get some in there and clamp it quickly before it runs out.
What about using a syringe with wood glue to squeeze the glue into the gap? You may need to drill a small hole to get the syringe into the gap (I'd drill from under the cabinet, so less noticeable). You could also use a piece of quad (similar to used between the side walls and base) to secure the back wall and base together - do that after you've pulled the pieces in with the pipe clamp.