In this case the balls (I only had 4 left after making the Strawberry cage) are used to extend the height of the verticals by joining two aluminium rods. The bricks across the corners of the base are for stability. I just hope we don't get any gales! And of course the net is longer than it is wide, so I had to fold the corners round and close up the gaps with some clothes-pegs. At least I chose the tasteful green ones, not the bright pink ones.
It may not look pretty, but I'm reasonably sure it will keep out the pesky Blackbirds. It only has to last a few weeks too, which is just as well in the circumstances, since it doesn't exactly exude an air of permanence!
Before this time next year I aim to acquire another Build-a-Ball set and some more aluminium rods, so that I can put together something a tad more professional next time.