Sometimes it's the little things that drive us all crazy...
I used a 1" "Button Plug" made by Servalite to cover my antenna hole in the fender. Just needs a dab of silicone on the underside lip to hold it down. It's black plastic, so it looks fine on my dark green car.
Servalite part #39008-550934.
At Orchard Supply Hardware, these can be found in the big, multi-shelf bin of miscellaneous indivially-wrapped fittings and hardware (and that lots of place have now, I think the entire unit is actually supplied by Servalite).
Another idea was the plug that's used to fill the hole on the top of a sink for the garbage disposal vent, but these are usually stainless steel, and I wasn't sure if I would also have a sudden urge to install a huge carbon-fiber airfoil as well.
And now that I think about it, what's wrong with having a radio in a race car? In my formula car, I had no choice, so maybe it was just a reflex to not have tunes in the 914, but hell, this car has other stuff my formula car didn't have, like a roof, so why not a radio? Discover that perfect song for the banking at Cal Speedway.... the Corkscrew at Laguna.... Turn 9 at Willow.... catch up on the news while I'm stuck behind that annoying guy in the yellow Alfa that always pass-blocks me from turn 6 to 9 at Willow...