I've used one of those stumpy flexible ones sold in car spares shops - OK, bits will rust eventually, but when they fall to pieces you buy another. Being flexible doesn't matter if it gets knocked now and again.
Any reason you need to mount it outside?
We've just purchased a cheap telescopic one and mounted in a tall locker - ok reception isn't going to be the best ever - but it is one less item outside and no thru-deck fittings either....
At the moment I have a length of wire inside pushed into the aerial socket on the back of the radio, and reception is not good. I will try an aerial internally next. Thanks for the suggestion.