Yeah I have. I used the black pan-head torx screws that are used to hold the arch liners so they are almost invisible at a glance. I'll grab a pic this afternoon :)

A couple of pan head stainless steel self tappers hold on to mine

Mine is exactly the same but with black screws :)

Thanks guys & girls +++

To save me crawling around and feeling the materials and behind, what's a good size and length to go through both layers and grip well without hitting anything it shouldn't?

10-15mm. There's plenty of space behind that surface. It only needs a small self-tapping screw - 4mm or so is fine.

Thanks. +++
The only suitable screws I have on hand are fairly short at 10mm thread so I'll give them a go first.

Job done, black 10mm screws both sides, nice and tight. :cool:

I think mine are drilled as per Oli18's original thread, and then cable tied with black cable ties, but I've also got the earlier 2 part grilles on each side so double the amount to fix *sigh*

I'm actually tempted to "double up" with screws bottom as well as top, or maybe outer as well as inner.