+xi would have a search over on the FSAE forums for foam attenuators, as a lot of the teams use foam for the job.
When i dont FS many moons ago we used a closed cell rohacell, i cant remember the density but it done the job. Have a look at the latest formula student regulations and they should give you a starter for 10. Also have alook on some of the rohacell seller sites like Emkay Plastics
Ah perfect thanks! Will have a dig
