Looking for inexpensive plastic butterfly/gate low pressure valve with low actuation force?

I need to find a valve, or some technique to heavily constrict the flow of water in a 4cm pipe. Pressure is low coming from a 2m cistern head above.

I have been searching in vain for a very cheap PVC butterfly valve with poor closed sealing characteristics, but with no luck. I did have a little more success with gate valves like this one

I am open to any valve technique provided it costs well under ten pounds in quantity.

Does anyone have any suggestions as to where I might find a very inexpensive plastic valve for this project?