I’m pretty sure the pink stuff I used is polystyrene (and I think Ed or Al mentioned that they used some as well).

When you sand it make sure you don’t use too much pressure. I was using 40grit for shaping and that works OK with light to medium pressure. If you push hard it will tear the foam and you end up with a gouge.

 

If you really want to make quick work of shaping your cowl get some Urethane foam from you local roofing supplier or florist (used in the bottom of vases to hold flowers). It’s a really light weight and brittle foam that you can sand my just looking at it (well almost). It can be shaped really quickly – but you need to be really careful with hitting or touching it – it dings really, really easily. Also, duct tape won’t stick to it so you do have to fill the finished shape with Micro or plaster / drywall mud and then sand that smooth and paint for release before fiberglassing.