1. Testors Matt varnish
2. Tamiya Flat Clear varnish
3. Mr Hobby Matt Topcoat
In that order of preference. Al the usual caveats about temperature, humidity and distance when spraying varnish apply but it works every time.
There used to be a more economical option from a local DIY paint chain, essentially twice the can size for the same price the but their matt varnish is almost impossible to find these days.