I would think its a good possibility the compartment is blocking a considerable amount of the signal. Do you get better signal indication with the door open?

I recently got called on to install a customer provided outdoor TV. It was from SkyVue. It had a similar comparment, but the cover was made from ABS, not metal. They considered this a feature because of the exact reason you mention. I could not test it as the client did not yet have a wireless DTV client to put in it. But it sounded like a good idea.