Just spoke to the club though, and they would really prefer if I got a proper dynafoam cushion for almost the exact reasons mentioned by astir 8.
Ask your instructor if the Eurostar has dynafoam seats and when he admits it doesn't, ask why your cushion needs to be dynafoam.
The original seat will meet the requirements and your cushion will offer extra protection.
That said, dynafoam would be safer.
It is not normally used in microlights, because it is too heavy, especially in aircraft that are anodized, instead of painted, to keep within the max ZFW.