A small brushed aluminum plate just a bit bigger than the foot print of the ashtray mounted to the fiberglass lip and supported at the front with two half inch aluminum straps leading forwards and up wouldn't work?
The support mount for the radio in my '65SC was somewhat similar and worked flawlessly.
The dash mount to keep it stable, the forward mounts for support, it ought to be fine, but photos of the underside of your dash would help, in case I can be of some help. Since I have the same ashtray here, I could use it to fashion the support plate for you, just need some photo reference of where it needs to be mounted.