How make round 5" air duct valves?

We moved into a large (for us) house, and we only use a few rooms. I managed to repair the furnace, but the first bill was astronomical. Beyond the leaking windows and lack of insulation, the problem is that, as it is, we have to heat the whole house. I've discovered a temporary method of blocking vent tubes to unused rooms (by stuffing the tube with a rag), but I want real valves. I've heard they are called air duct dampers. 
My vents are on the ceiling aimed down.  
So, I've seen these check-valve 5" ones but I haven't found any that would open and close manually. 
But this doesn't seem like it would be difficult to make. I'm toying with the idea of using a cone-shaped "lid" with some kind of springy cord holding it in.
Does anybody have any ideas? 
How do I make a manual (or electrical solenoid, I suppose) operated valve for a 5-inch round air duct?

DavidE3414 days ago

In the USA they are called diffusers and are readily available in most home centers or HVAC supply firms in 5" (or smaller/larger); can even get them adjustable like this:

I have the same problem here and had it in my old house as well.
All vent openings for heating and (evaporative) cooling are just open with no flaps or flow adjustments possible.
Heat goes out the cooler tower like a chimney and of course the rooms you don't really want warm, like a bedroom, will receive the most heat.
My first gas bill here was about $200 higher than what I feared and maybe my simple fix can help you until you find what you are looking for.

The vent outlets for the cooling in the ceiling have smaller squares with flaps but they can't be fully closed.
I simply cut some cardboard to match and placed it between the frame and the smaller outlets.
Since it is a solution for several months I was not too bothered to cut a few squares and use a step ladder.
Different story though for the heating outlets.
Usually they have a metal flap where the duct pipe connects which allows for flow regulation.
In this house there are only metal stumps left where these flaps were attached years ago.
My outlets have a flat bottom with the pipe in the middle and the flat part either side is more than the pipe diameter.
So I use two pieces of carboard and two rocks to fully close or partially open the air vents.
Since noone uses a flashlight to check my vents I again can't be bothered for a better fix in a rental.
However if you want a more permanent and flexible option check your hardware store, often you can get the part for your outlet with a manual operated flap.
Around here this box is around $30 AU if in plastic and about 50 for metal.
Not sure if you can get them small enough but for the round heating outlets you get the system with a screw flap, or better two flaps.
When you twist the center the flaps move up or down and these do fully close.
If you can find them in the right diameter it as simple as cutting the excess you don't need off and to glue the rest into your pipe opening.