Is there an antenna hole cover?

Not sure there is a "proper way" Originally non-radio cars did not have the antenna holes.

You could use an electrical box hole cover available at hardware stores (have used these to cover windshield wiper holes) or you can look for a mid 80's Mustang antenna hole cover (rectangle flat plastic piece) by looking on ebay

 

Mike,this is not meant to be insulting but the painful answer to your question is some creative welding and body work or a new fender without a antenna hole. The damage is done. Maybe not as "proper" as factory but the next best way would be as Snakepit described. Bob

 

I don't know of anything specifically for the antenna hole, however if you want something to plug the hole without welding and repainting, which as Bob mentioned would be the best way to do it, you could use any old hole plug and have it painted to match.

There are a lot of industries outside of automotive that use plugs, so finding a plug that would work should only be as difficult as finding the right size plug in a suitable material to receive paint. Try here:

http://www.surplussales.com/MetalPlugs/MetalHolePlugs.html

Hope that helps,
Josh