Hmmm, tough one. Well, I would use the very biggest fan possible, the 16, and fab up some aluminum or steel brackets to make the fan stand off of the condensor. You could make some strap type mounts (like 2 of them) to run horizontally across the front side of the condensor, and mount your pusher to those.

Pushers are generally less effective than a puller, but if you go with a really good quality fan like a SPAL, you should be ok, I think.

Don