Not sure about an NSX, but I removed a power antenna assembly from a Miata and found what's called a sink blank at Lowe's. It's the chrome caps that cover up unused holes in a kitchen sink. They are made out of spring steel, flared at the bottom to hold them in and chrome on top. I just roughed up the chrome with sandpaper, sprayed on primer and then sprayed on touch-up paint to match the car, popped it in and it looks like a factory blank. If you go this route, make sure to get the blank for a stainless steel sink as it will be made to fit thin metal versus a porcelain sink which would rattle around. Also make sure it will actually fit an NSX hole but they sure look the same to me.
I recommend using a silicone sealant to place under whatever you mount to the hole since weather will weaken most bonds. This will keep water from entering into your trunk if you removed the tube that normally funnels water down and out of hole where the antenna is.
Thanks guys for all your help. I found a solution with a BUTTON PLUG from the hardware store. I took the oem antenna rubber cap and match its size to a button plug. I found one perfect fit and I'm sure not even one drop of water can get in there:smile:
Just a heads up, you'll need that rubber or silicone barrier to not only seal up the hole, but also to separate dissimilar metals and prevent galvanic action in which one metal will corrode the other (i.e. steel and aluminum). http://en.wikipedia.org/wiki/Galvanic_corrosion