So I took the advice of a few posters and mounted the gaitor ring underneath. In order to have the tray not flex when fastened down, I made a second gaitor ring to use as a spacer using the original metal one as a template out of some 5mm thick plastic I had lying round the garage and then mounted it between the tray and the metal ring. Essentially this makes a 'tray/ spacer/ gaitor ring/ rubber gaitor/ carpet/ body shell' sandwich.
Some photos:


Painted the screw heads with some satin black for a neater finish also:

And Mini Cupholder 2.0 in all it's glory:


Cheers!