I had the same issue, I found a few of the front plates on ebay, but they wanted way to much for my taste (with shipping 75.00) so I made my own out of aluminum...a good stereo shop can hook you up for less
They sell them at best buy i got one about 2 years ago and the price was fairly cheap. it looks good in mineMy 92 vert had one in it when I bought it. It has the hole for the stereo on top with a storage cubby under it. It looks factory. I know Ive seen them for sale somewhere when I was searching for the correct wiring diagram for my car. I cant remember where though.
Edit: This looks like the one in my car.
Ford Mustang 1990 1991 Radio Stereo Install Dash Kit - eBay (item 390218431321 end time Aug-12-10 20:40:23 PDT)