I've used 2mm/.08" Sintra on my last 2 helmet builds and cardboard on my 1st. I have to say that sintra is much easier to work with by far in my opinion.
Sintra is basically a thermoplastic so it bends with heat. A hairdryer will do the trick. I've used a heat gun with it too, but...keep in mind that sintra will hit a point where it shrinks and is basically useless. I've ruined many dome pieces this way.
It's much easier to cut than cardboard. It's also not as stiff so you don't have the bending issues.
Use a good quality super glue/CA glue. I find that the Krazy Glue liquid is great. It bonds instantly, but if you make a mess with it then you have to go back and hold for 20 seconds or so to get a bond. With cardboard be prepared to do a lot of clamping and waiting.
You will need to catch a fiberglass tut on youtube because you will want to fiberglass the interior if you plan on wearing it much. It's easy and smelly.
That's my 2 cents. If you need to know anything else just ask. I've done everything the wrong way at least once.
