Tightening patterns?

[Mr. 16 gauge]

I'm looking for ideas on how to tighten up patterns on a cylinder bored smoothbore....I know the basics of 'less powder, more shot", but I have heard other things, such as wrapping the shot charge in leather, ect.....I'm looking for ideas like those.
I'd like to stay as traditional as possible, please (i.e. no plastic shot cups! )
Thanks............