Round Air Conditioning Vent Tips/hints

I've got a 39 chevy and I want to put round vents in the dash on the far ends. The dash in this area has a slight curve towards the door. My question is what is the best way to create a good mounting surface for the vents to fasten to. My vents have a billet face that un-screws from the round plastic extension. They are meant to have thin metal sandwiched in between.

My idea was to fab a short extension from a tube and attach a small lip on the end so the billet face and plastic tube have something to sandwich in between.

Does anyone have pictures of thier install or a link to another thread?

Thanks.

Would it work to install a tube in the dash with the end of the tube contoured to fit the dash and the duct in the tube with a thin piece of metal sandwiched in between to hold everything in place????? or, maybe just a plate on the inside of the dash slightly recessed from the face of the dash to give you a flat mounting surface??????