Hallo people!
In the last few days I've been working at my fantasy helmet, and since it's the second I made, I wondered if such a topic would be of interest here.
Hence I'm going to show you my two helmets, the historical and the fantasy one.
Here we begin with the historical one.

Front view.
Right side.
Back view.
  Internal view.

This is my hypothesis of a 8th Century b.C. linen cap. It's made of 4 layers of linen glued and sawn together.
I've used carpenters glue and beewaxed cotton wire.
Various sources show this kind of helmet, or a very similar one. Sources I've used are:
Homerus' "Odyssey", a 8th Century b.C. copper cap find in Northern Italy, a 7th Century b.C. cap made of bark found in Southern Germany, representations in pottery and stele found in Region Emilia-Romagna, Umbria and Marche and dating from the 6th to the 3rd Century b.C. Plus a lot of modern Authors.
My cap will equip a Picenian warrior of modest wealth, who fought between 8th and 7th Century.
Unlike Homerus' description, my cap is a bit thinner, but it's still quite hard thanks to the glue.
I won't rely on it, though: it may absorb part of a blow, but it'd hardly save my life.
Which makes an interesting light on the role of active defense in ancient battles fought by my ancestors.
Greetings,
Mauro.

Topic added to the Index!
Greetings,
Mauro.

Ahahahaha thanks for the second 
Regarding the first one, I agree with you that a poor one is better than nothing.
Always Homerus tell us of Psilòi (Greek light infantry, almost always poor pastors) protecting their heads with felt hats, and using a thick felt mantle as shield.
I can see the relationships between sheep herding and felted wool gear, it makes sense!
I used a 100 years old linen sheet I've found in a church, and I think I'm going to harden it with beewax or a bath in water+salt.
If it doesn't harden, I'll make another cap with felt and will leave this one as an experiment.
Another scientist has made a leather cap like this, but it's just for display so we don't really know how effective leather can be (I bet it would work great).
Greetings,
Mauro.

The minimum i would use in a battle,and even poor folk could make for themselves is a 1" thick felt inside,with a 1/4 " thick hardened cow hide outer.