Preventing draft under door
You might try one of these, if it could fit. They're pretty cheap as well and basically like a double sided "door snake".



Does the storm door itself have a sweep along the bottom? We bought an old house recently that has original doors to the outside. The storm door that the past owners installed works great as it has a sweep that 'scrapes' along the concrete outside until it seals in. If it's open the wind just blows around every crack of the door, but when it's closed you can't really feel anything.