You could try putting the tray outside - although the danger then is that every passing cat will start using it as a public toilet. Before you know where you are there will be cottaging, drugs etc etc. Then you'll have to put in some blue neon lights. It will be a nightmare
Or you could just sprinkle a bit of cat litter on top of a handy patch of earth to encourage them to go - but use the biodegradable litter stuff.
Our latest cat (now 9months) used to come back inside to dump, but as she spent more and more time outside she did start pooing outside of her own accord eventually. I think it's just a matter of time.