I agree with Hyacinth in the fact that you need to do something asap before anything happens - which I'm sure your aware of.
I'm in the process of buying an electric containment fence online (wayyy cheaper) to keep in my houdini furkid. She is not aggressive when she gets out but we live near a national park and I'm paranoid once she gets in I'll never find her. Have you thought about an extra strong electric shock one? Sounds horrible but its more of a shock then a pain.
I think the enclosed section would be the next best if it could be modified (as Minibull suggested) to have an outside section to. Could also be alot cheaper.
As for why shes aggressive I cant help you on that one, it sounds like shes been socialised heaps so maybe its just her personality. Does she have any other dogs with her? Or any dog playmates?