problem with a doggy door is that you won't have control of the dog when/if it goes outside in the middle of the night. Could be overcome if the door opens into the penned yard. I reckon you could adapt a dog door to fit into the security door providing there is enough room for it to swing inside. Would need a slide lock for travelling. And this means sleeping with the main door open - might be a bit cold.We take ours out last thing at night and then first thing in the morning and have not had a problem with 'accidents' inside.

If you must keep it very small. Realise that possums are good at pretending to be doggy's.