Take a look at prefab tool/garden sheds, either metal or plastic like the ones sold at Costco. They come in sizes and hold up very well to rough weather.

Since bikes are light, either anchor the shed properly, or put some cinderblocks or something heavy in the inside corners.

Warning, once you put one of these up, the wife will find all sorts of stuff to store there, and your bikes might be out on the street again.