Hi Hornet,
I have kept my diamond outside for many years and she is now 9 years old. I am informed that this is a good innings for a diamond and she still has a lot left in her. She is in an aviary that is 3m x 2m x 3m (pic attached)high. The main issue with outdoor keeping for you would be letting them escape from the heat. If you use a fridge make sure the front doesnt face north and is in the sun. You would have to keep it undercover and provide UV another way.
As the guys have said, build an aviary. Make sure they have access to 8 hours of sunlight but also an ability to escape the heat. You shouldn't need any heat source as these snakes occur locally anyway.
Any yes, buy a big one. Keep looking, you will get one cheap.
Oh, btw, they tend to lose their tame when they are outside as well.
Cheers
Pete