What method are you using as a base for the model, foam or paper/resin?
I think either way you just want the regular non-fibre, non-metal-additive type filler/bondo
I've only got the experience of 2 helmets, but "cool" isn't really something you can control inside the helmet. Keeping fresh air available IS however and will make a huge difference especially if you are hot inside. Thankfully a lot of Halo helmets have vent looking things in the design right at the front. I put small fans behind these in my mk5. You can use PC fans,try to track down the smaller 30mm ones. PC fans are quieter than the scroll like blower fans which you can also get very small. this noise makes a diffference when inside a small confined space with your ears. If you don't have the spots to incorporate the vents in the design of the ODST helmet, then even some holes drilled up through the underside of the 'chin' of the helmet will help draw fresh air straight up to your mouth and nose, they also most likely wont be seen unless you stare at the roof and show off the neck hole of the helmet