Thanks
I took off the under tray and gearbox, layed the chassis on the brass and scratched around it. Then just cut it out with a slitting cutter on my dremel, lots of light slits until it goes through. Only took about half an hour, including fileing, sanding and polishing the edges. The rear of the undertray was a bugger. I had to use a mini blow torch and LIGHTLY play it over the end of the tray until it went soft enough to bend to get back under the rear bumper. If you don't bend it, the tray will 'bow' when the bumper is put back on.
0.023" adds 138g, and sheet brass is cheapish when bought from
www.metalsmith.co.uk