I put in the banana vent grills with Permatex black ultra. I used a small hex head screw from the back through the grill and three pieces of wood (top, center and bottom) to secure them while drying. For those that haven't done this yet, this is done easiest when the rear splash guards are not in. Thank fully I made access panels to get to the rear light harness so I was able to work it from there.
Unfortunately the Permatex bled through the front and is visible in places.
I see that Permatex makes a remover for their products but not sure if that would ruin the paint?