Krylon makes an ultra flat spray(Walmart). But if it goes on a hard surface and it gets scuffed it will appear as a shiny spot.
I heard Pittsburgh paint makes a real decent flat black that will not shine when scuffed, that is what many decoy painters use on their wood decoys.
If you are paint polyethylene plastic heads-nothing is meant to stick to the plastic. So use a plastic primer like that used on plastic car bumpers after lightly sanding the platic with a soft sponge sandpaper block.
I hope this helps.