You really can get away with mounting one of these to the side of a wall:
But then you have to wire it with a 5 volt power supply somehow. I got away with just mounting it inside a vent so that the actual pi is inside the garage and the camera its just clipped to the housing.
You can go overboard with a case that costs more than the pi itself:
I kinda like this guy's solution:
I like the last idea. You could probably make an argument that it would be more efficacious as a deterrent (assuming that is part of your end game) if it is clearly visible.
Maybe add a sign that says, "You are on camera. You can smile...or not, I do not give a %&*$."
If I am honest, I am so paranoid I would probably have fifteen of these stupid things inside and out if I was not aware of my neurosis.
You need to figure out how to use a battery strapped to this thing to get an idea of how long it would run before it had to be changed. Make it so.
I think a 10000mah battery will run it for a couple days.