Found one - but it’s not XTR, sorry.
Here it is, on the right, with genuine M900 XTR on the left.
Could be cut to fit, but wouldn’t have the extended cover as the genuine item has.
Looks the same here;
But isn’t;
This is how the genuine one sits on the XTR mech;
And how the non-XTR would sit, it would need a bit more of the cover cutting out;