I second Chris, and add the following.....
Personally, I haven't had a lot of luck with Testors acrylic flat, even airbrushed on. And since you need a rattle can, your choices are going to be limited. That's why I'm seconding Chris and voting for Testors Dullcoat. Just make sure the acrylic base is well-cured.
OTOH, If you prefer to brush on your flat coat, IMO Vallejo Matte Varnish would be a good choice IMO.