I don't know if it is the same in the UK as it is in the US, but the appeal of experimental aviation (every Cozy is experimental) is that there are no specs or requirements around parts like this.

If you want to slap 15 weed-wacker engines to a wing you made from styrofoam and call it an airplane, the FAA will not stop you.

I'm oversimplifying, a bit, but less than non-pilots might think.

In other words, the engine maker probably has some thoughts about how that piece should be made, but the FAA would have no problem with you installing it on an experimental.

The spec is fiberglass, which has better thermal resistance.