In a previously owned RIB (an Avon SuperSport 345) I just bolted an adjustable mount directly to the transom. The only down-side is that it was not as high off the water as would be ideal, and whenever one broadcasts, one is exposing oneself to the EMF radiation from the antenna (if you worry about such things). If I did it again on a larger RIB, I'd probably install an extension blank of 1.5m or so, then the antenna in order to get more height.
And here was the "business end" of things:
