I made my own panel for my portable AC fan. I don't like the flimsy panels they come with, so I cut two pieces of wood, and added a couple layers of Siless (Car insulation) to dampen the sound and help with insulation. However I didn't like the final look of the wood.

This panel simply glues on top of whatever panel you have. Mine has a final cut to leave some space where the panel is behind the frame at the bottom. You can change the diameter of the hole, the position, and the size of each part to fit your needs.

The template file allows you to verify the position of the vent hole, or help you guide where to make the cut.