The last time I used them I sealed around them with Isopon P40, that's the car body filler with the chopped strands of glass mat embedded in it. But it all depends on what the hull is made from, mine was a grp hull, if its a styrene or timber hull there will be different methods. But these methods will generally use an epoxy glue of some description.
Maybe Tug Kenny will happen along and let you know how he sealed his schottels into his Seabex One.