Hey Dave ,
I just had a brain fart here , but what about making a cone out of metal , with just a slightly smaller opening you need at the rear of the grille shell and hammer form it to the size and shape to your needs , and then the front area for the grille you can cut any shape & hammer form the hole to your desire .

May be more work then you want , but like I said ,it was a brain fart .......LOL

Spray