What can I use to make the fluffy but on a Roman Soldier's Helmet?

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I've been making a costume for DS1.

The helmet looks a bit <a class="break-all" href="http://www.google.co.uk/imgres?imgurl=school.discoveryeducation.com/clipart/images/roman.gif&imgrefurl=school.discoveryeducation.com/clipart/clip/roman.html&h=389&w=421&sz=6&tbnid=A7fCfXxxdtXKbM:&tbnh=115&tbnw=125&prev=/images%3Fq%3Droman%2Bhelmet&zoom=1&q=roman+helmet&usg=__fWGMGVZJ5WInYrbpGOOzNNhkhL0=&sa=X&ei=RnXpTOi2HqOAhAfBnfEP&ved=0CCMQ9QEwAQ" rel="nofollow" target="_blank">like this and was made by attaching cardboard flaps face guards to a plastic policeman's helmet and covering the whole thing in foil, but what can I put on top?

A soft broom head?

I used a broom head for a class assembly, did the job!

Crepe paper cut into short lengths and stapled between two pieces of foil-covered cardboard might work.

tinsel?

Short strands of wool / string dipped in either PVA or a strong sugar solution to make them rigid.
Crepe paper will work if you fold it into a narrow concertina front to back - you need to wodge enormous amounts tightly together to get it standing upright.

how about cardboard, painted red, fringed so it stands on end.

If you fold crepe paper so it a strip (as long as you need it) by about 6 inches

Then fold it in half, fold back the ends so you have an inverted V with little flaps, the flats should glue to the helmet

Yes go with red crepe paper. I have an old jane asher book which has a great knights helmet in it to make. She uses some lengths of florists wire with rectangles of crepe paper folded in half and glued around the wire. Then just snip along to make a feathery effect. The wire will keep it upright