I have seen flapper valves that install in the line (on the wall on the inside of the house where the vent duct exits). These flappers work using the air pressure from the dryer operation. Therefore they are automatic and don't need to be opened and shut by you.
Henry is right -- they will collect lint and may be a fire hazard.
The interior flapper should help. Perhaps your real problem is that your house is drafty and the warm air escaping from the upper floors of your house is creating a suction that draws air through the crummy dryer vent louvers on the outside of your house.
You can get better louvers or a flapper (interior or exterior) that seals better. Go to a good home center or plumbing supply store and tell them about your problem. You don't want to create too much back pressure for your dryer blower. --Billy