I went a different route in the end.
Took off the aerial to find a square hole :/ Figured I wouldn't find a grommet anywhere easily to fit it (it was about 12mm by eye I would say)
I stuck a piece of firm plastic to the underside (leaving a small pocket which allowed filler to spread out around the edge of the hole)
Then filled it with epoxy filler, making sure to squidge it into the gap under the hole for a better grip.
Sanded it down and sprayed a very small area around the hole where the aerial base had chipped the paint a little (also filled)
Going to let the paint dry overnight then sand it down and give it another coat, then blend the edges.
It's such a small area, and not in a particularly noticeable place, so I am happy to have the small square not being a perfect match, but it is damn close

.
In my eyes, much nicer than a big black piece of plastic sticking up end being more noticeable
